Tom Steyer is the kind of all-American overachiever who seems to exist only on paper: number one in his graduating class at Phillips Exeter Academy, summa cum laude and captain of the soccer team at Yale, a Stanford MBA who became a superstar at Goldman Sachs and then went on to create the world's fourth-largest hedge fund, turning $8 million into $30 billion in about 20 years. His personal fortune is estimated at $1.5 billion.

So it's a bit startling when the human résumé comes bounding out of a narrow brownstone in San Francisco's Pacific Heights on a cool spring morning, pale legs exposed by knee-length shorts, wearing white socks with black trail-runners. With his flop of copper-blond hair and his shaggy enthusiasm for a 7 am hike, he seems less like a Rockefeller than a golden retriever ready for his run.

"Good morning!" he booms.

As billionaires go, Steyer doesn't conform to type. He drives an outdated hybrid Honda Accord and wears a cheap Ironman watch with a Velcro band. His "briefcase" is a worn-out canvas bag exploding with papers. In front of his house is an exotic eco-sculpture, built by a bohemian stonemason, that filters a fish pond by running the water through an elaborate cascading planter. "We're the only people with hydroponic gardens in the state of California who don't grow dope," Steyer quips. "What a bunch of idiots!"

In San Francisco, of course, inconspicuous consumption and a small carbon footprint are common measures of status. But as Steyer and I huff and puff down a nearby trail, through shady cedar groves to a promised bay view, he lays out a vision for his life's work that allows for at least one ostentatious trophy: to be the man who saved the world.

Since early last year, the 56-year-old has been using his vast fortune to wage political warfare against climate-change deniers, eco-antagonists, and oil-industry sympathizers. He started a super PAC that goes after candidates who support the building of the Keystone XL pipeline, the joint American-Canadian oil project meant to help funnel millions of gallons of dirty fossil fuel from Canada to the Gulf of Mexico for sale around the world. In Steyer's view, the oil from the pipeline would produce enough carbon to send the world sliding into irreparable disaster.

"We don't have time," he insists. "This is an urgent issue. If we produce this stuff in Canada, it's a 50-year supply, and I guarantee you they'll find more. If we do nothing, we're dead! We're toast!"

Climate change isn't something scientists argue about nowadays – 97 percent of them agree it's here and it's our fault – but politicians are another matter. Republicans, by and large, believe the threat is overblown. And even some Democrats argue that, despite the deleterious effects on the Earth's atmosphere, the Keystone XL, the license for which President Obama will approve or cancel later this year, could goose the economy.

Steyer compares this thinking to a heroin addict's logic. "You know what, I'm going to get off heroin, but hold on just a second because I just bought 10 pounds," he says, sarcastically. "And when I'm done with that 10 pounds, I'm off! I'm done!

"Bullshit."

For Steyer, the ticking time bomb of rising carbon dioxide levels – the destructive output from burning fossil fuels – is so loud that it drowns out all other issues. "There will be droughts, there will be floods, there will be oceans rising," he promises. "So there's going to be hundreds of millions of people with no water. Do you know what people with no water will do?

"It's like in Chinatown," he says, "where he turns and says, 'Mr. Gittes, under the right circumstances, a man will do anything.' "

Steyer believes we have to act now – not 10 years from now.

With his square jaw and boxer's nose, Steyer has the disposition of an Irish pugilist, an aggressive financier animated by a sporting esprit de combat. As an investor, he made his billions by taking stomach-turning risks on distressed assets in volatile market conditions, such as the Thai baht crisis in 1997 and the dot-com bust a few years later. He's taking the same approach to Washington, betting on the ultimate distressed asset, the Earth, in hopes of reaping the not-insignificant return of sustainable life on the planet.

And with that, of course, comes a powerful legacy as the man who forced the issue into a recalcitrant political system and saved us all. "This is something that's important and big, right here," he says. "I want to win this."

Since stepping down from his firm, Farallon Capital Management, at the end of 2012, Steyer has hired a cutthroat political consultant and financed attack ads and armies of door knockers in Massachusetts and Virginia to defeat two candidates who supported the pipeline, all through a nonprofit political organ called NextGen Climate Action Committee. They weren't exactly long-shot races, but Steyer managed to put himself on the map with around $10 million and a sizable dose of chutzpah. In Massachusetts, he demanded the target candidate renounce his support for the pipeline by "high noon."

Bill McKibben, the environmentalist writer and activist whose 2012 article in Rolling Stone galvanized Steyer's political will to fight climate change, says Steyer has "made it very clear to the political class that you don't get to screw the planet and get away with it scot-free. There's a price to be paid there."

The unmitigated flow of money into politics isn't everybody's idea of a high-minded solution. But Steyer takes a zero-sum view: If moneyed oil interests are paralyzing our political will to confront climate change, then moneyed antagonists are required to counter them from the left. As he hikes down the trail – "Good morning!" he says to every passing hiker – he spends a fair amount of time contrasting himself to his analogues on the right, especially the Koch brothers, the plutocrats famous for backing pro-business candidates with their group Americans for Prosperity. Steyer says coal-industry baron David Koch is "taking the most incredible risk that I've ever seen someone take, of going down in history as just an evil – just a famously evil – person!"

Measured political rhetoric is not Steyer's style. But when we finally arrive at the plunging cliffside, with a stunning view of San Francisco Bay and the Golden Gate Bridge under a vaulted sky, the moneyman goes uncharacteristically silent. Standing near the edge, he turns to me and grins, his arms lifted in triumph, and presents the breathtaking horizon like it's the last slide in his personal PowerPoint presentation.

"Yes!" he declares.

After the hike, we climb back into his wife's Toyota hybrid and Steyer plays some Rod Stewart on the radio and idly recites Rud­yard Kipling: "On the road to Mandalay / Where the flyin'-fishes play / And the dawn comes up like thunder / outer China 'crost the bay –

"Good grief!" he says. "They're blocking my street."

Steyer is taking us on a detour to show off a new house he and his wife, Kat Taylor, just bought near China Beach, a multimillion-dollar fixer-upper that looks out over a cliff to the glittering waters of San Francisco Bay. "Do the words water amenity mean anything to you?" he says, jovially.

Back in the neighborhood of Pacific Heights, at his old brownstone, Steyer shows me the oil paintings adorning his walls ("These are all living California artists," he whispers) and proceeds to the kitchen for some organic tea. "Smell that lemon," he says, holding up a Meyer, freshly plucked from his garden. "Sometimes I just squeeze them on my hands."

On the walls are pictures of his four grown kids as towheaded children – "boy, boy, girl, boy," Steyer recites – and his many beloved dogs, living and dead, all of whom have coffee mugs made in their likeness. His wife, an activist and lawyer who has five tattoos and the toothsome charm of Sally Field, drops in for some casual gossip about a financier who Steyer says has a "questionable business model." The house has the homey, lived-in feel of a guy who used to be a mere millionaire. Or maybe just a billionaire with limited pretensions.

Not that he's laid-back. Steyer's idea of a Christmas vacation was taking his wife and kids on a five-day hike to the peak of Kilimanjaro two years ago, where they gobbled acetazolamide pills to avoid altitude sickness. (Steyer saw the trip as a convenient diversion: His oldest son was doing nonprofit work in Tanzania at the time.)

Though he has choice words for David Koch, they're both part of the billionaire boys' club, and Steyer is an apologist for the winner-take-all world of capitalism. When I bring up the controversial role of Goldman Sachs in the 2008 financial crisis, when then treasury secretary Hank Paulson, a former Goldman CEO and a pal of Steyer's, appeared to favor his old firm during the multibillion-dollar bailout, inspiring the 99 percent backlash, Steyer admits that his former employer "got deferential access and deferential outcomes, and that anybody who doesn't get that is a fucking idiot."

It's a surprising populist outburst coming from a former Goldman Sachs man. But in the same breath, Steyer says Paulson did a good job and, all things considered, everything worked out pretty well for the U.S. economy. And anyway, "that's the power of finance in the world," he says, shrugging. "You're fighting city hall."

Steyer is himself part of the city hall machinery. When it comes to climate change, however, he's the power willing to speak truth to the other power. "People are scared to tell the truth," he says, lowering his voice to a serious whisper. "And you want to know something? They're not stupid. Because you get punished for it."

Are you afraid? I ask.

"I'm too stupid," he says.

Steyer, of course, is far from stupid. Intensely analytical and feverishly focused, he is an overachiever both by disposition and by upbringing, spending most of his life wired into America's most elite institutions. He came from a well-off family in New York City, the youngest of three sons, his father a corporate litigator at Sullivan & Cromwell, the white-shoe law firm that has represented Goldman Sachs for decades. (The firm also represented Exxon, and among colleagues the elder Steyer was known as "Mr. Oil.") Growing up on the Upper East Side, the Steyers had a housekeeper and a nanny, and they summered in Nantucket, but Tom's mother, a progressive and principled woman whose tough-love mothering included washing his mouth out with soap when he swore, was a socially minded educator who at one point taught felons to read in a Brooklyn detention center. "I once asked my mother, 'Who are we? Who is our family?' " recalls Steyer. " 'Well,' she said, 'we've never been rich or powerful, but we've always been well-educated and we've participated in our community.' "

Steyer went to a distinguished Manhattan boys' school called Buckley that in the 1960s taught boxing in gym class. "The whole key to a fight is throwing the first punch," observes Steyer.

When he graduated with honors from Buckley, his older brother Jim recalls the headmaster encouraging the class to "hitch your wagon to a Steyer!"

At Exeter, Steyer continued his competitive march: first in his class, chairman of the student council, lettering in three varsity sports. He went to Yale, where his father had been editor in chief of the Law Journal, and became captain of the soccer team while earning degrees in economics and political science. On the soccer field, he was a sweeper, tasked with knocking down opponents. "I think I played soccer every single day, 365 days a year," Steyer recalls.

His interest in investing came early. After spending a summer working on a cattle ranch in Nevada, he came home and bought stocks of agricultural equipment manufacturers, observing that they made the lion's share of the profits in ranching. "That is not a normal reaction to two months on a horse," he confesses.

After graduating from Yale, he spent two years working as an analyst at Morgan Stanley, slogging through 90-hour weeks. To blow off steam, he played pickup soccer matches in Central Park with some particularly aggressive players. During one game he was kneed from behind by an opponent, severely injuring his right leg. He still walks with a slight hobble.

Steyer got into both Harvard and Stanford business schools, but was convinced by his brother Jim that there was a better life to be had out West. It was a fortuitous decision: During his second year, he met fellow overachiever Taylor, a Harvard grad and a former track champ who was simultaneously getting law and business degrees from Stanford. It was a match made in Ivy League heaven.

While at Stanford, Steyer says, he saw a listing for a job at Goldman Sachs, working under rising star Robert Rubin, the future secretary of treasury in the Clinton administration. Steyer was qualified for the job, though it could not have hurt that his father was a litigator at Goldman's preferred law firm. When Steyer told Rubin he wanted to spend the summer working in politics before he started at Goldman, Rubin helped him get a job with Walter Mondale's presidential campaign in 1983.

It wasn't the civics-class version of politics Steyer had naively dreamed about. When he set up a successful fundraiser for Mondale in Seattle, his boss stole the credit and wouldn't allow Steyer to attend. "It was a little shocking to me that it's kind of a tough, cutthroat business," he says. "I never thought of it as a business."

At Goldman Sachs, Steyer was part of a small, elite division conducting, among other things, risk arbitrage, betting on the new wave of corporate mergers and acquisitions. The alumni of Steyer's group would become Wall Street titans, a powerful network whose investment models foreshadowed the rise of hedge funds, financial deregulation, and the monster profits of the next corporate age – including, to be sure, the collapse of the economy in 2008. But after only two years, Steyer decided to leave Goldman and strike out on his own. Taylor, a native Californian and an avid equestrian, yearned for the wide-open spaces of the West, where there was no shortage of opportunity for a Goldman Sachs man adept at the hottest investing strategies.

He named his firm Farallon, after a shark-infested shoal on the California coast, and it quickly earned a reputation for take-no-prisoners investing, raking in millions, and eventually billions. Steyer picked through the books of flagging or collapsed companies, made sophisticated bets on them, and often conducted ruthless campaigns, legal and corporate, to force their value up and reap a massive return.

As he approached 40, however, Steyer had what he has described as a revelation. He became more involved in the local Episcopalian church, the religion of his mother (his father was a non-practicing Jew), and sought celestial perspective from his myopic financial concerns. "I think I was on this very straightforward escalator – grammar school, high school, college, get a job on Wall Street, kind of everything leads to the next thing," he says. "But at what point do you get to step back and say, 'I'd like to take a broader view of my time on this planet?' "

His oldest brother, Hume, a lawyer in New York, says Steyer worried that his wealth and success had put him in a room full of mirrors in which he looked pretty damned good – but maybe a little too good. "Tom got a pretty healthy dose of ass licking, and he was frightened by it," Hume says. "And he tried to cast around for a new way to think about things that would put him in his proper place."

Steyer was a very wealthy man who wanted a legacy other than his bank account. "I really don't want the highlight of my life to be my success as an investor," he says. "Genuinely. My idea of death would be that person who is still telling you about that goal he scored in 1974."

For a second, there is a comic beat. Wait, I ask, did Tom Steyer score a goal in 1974?

"Yes, I did," he says.

Steyer's passion for climate change didn't strike in a bolt of lightning. "I don't think there was one moment," he says. "My feeling was, I kept watching it and waiting for America to engage."

Not long after George W. Bush was elected president in 2000, Steyer met Chris Lehane, a former Clinton White House operative known in D.C. circles as the "master of disaster" for his crisis management during the Clinton years. Lehane became Steyer's informal advisor and sounding board for a speculative entry into the political arena. In 2003, Steyer briefly considered running for governor of California during the infamous recall election of Gray Davis, in which Arnold Schwarzenegger eventually defeated a woolly cast of candidates who included Gary Coleman from Diff'rent Strokes.

"We didn't think he could win," says Hume, "because it was going to be a short campaign, and Tom didn't have any name recognition, and he would have been tagged with the rich-hedge-fund-manager problem."

The year Bush was reelected, Steyer found himself on the receiving end of left-wing ire when student activists at Yale began investigating how exactly he made his money. During an internecine political battle with Yale management, the activists learned that a sizable chunk of the university's endowment was in Steyer's hedge fund. Consequently, they began a media campaign to embarrass Yale by attacking Farallon, accusing the firm of investing Yale's money in companies with anti-labor and anti-environment practices. They were soon joined by Steyer's other alma mater, Stanford, which also had millions tied up in Farallon. The anti-Farallon group started a website called unfarallon.info, a repository of negative news on Tom Steyer, and organized public protests and political theater events, one featuring a giant papier-mâché globe and a woman wearing a sign that read my name is farallon capital management, who used a hammer to smash parts of the Earth where Farallon allegedly caused ecological damage.

Steyer was stung, angry at being singled out as the go-to bludgeon to attack Yale. "Tom was so angry because it was a completely unfair smear campaign," says his brother Jim. "It woke Tom up, I think, to the fact that he could be used as a political pawn, and people would play politics even if it was irrelevant."

But in truth, the critique of his investments was not only relevant but also just the tip of the iceberg: As early as 2000, Steyer's fund had invested heavily in the energy companies developing the Canadian oil sands – the very companies he is now fighting.

Behind the scenes, Steyer and his wife were embarrassed because they were personally committed to social justice and environmental causes. Taylor was starting a community bank in Oakland and had begun to dabble in grass-fed, hormone-free cattle ranching (buying an 1,800-acre property in Pescadero, California, which they dubbed TomKat Ranch). Now Steyer's hedge fund compromised their moral purity. "It was a little flare going off in our minds," says Taylor. At the time, she says, they both thought, "One day we want to be totally aligned. We haven't earned that moment just yet, but we're going to get there."

But first he needed to find a new career so he could extricate himself from the conflicts of his fund without having to shut it down.

The next year, during the 2006 midterm elections, Steyer teamed up with Lehane and others for a Democratic fundraiser in San Francisco to support his friend Nancy Pelosi in her congressional reelection campaign. Taylor enlisted Jackson Browne and Bonnie Raitt to play at the party, later stealing the limelight by singing Billy Joel's "Summer, Highland Falls" a cappella.

Afterward, Bill Clinton stepped up to speechify and marveled, "How can I follow this?"

The Steyers were a hit. "Tom realized, 'Oh, my God, I really have the ability to influence things,' " says Jim Steyer. " 'I'm a player.' "

Steyer became a top donor to Hillary Clinton during the Democratic primaries for president in 2008 the following year. When Obama won, Steyer's name circulated as a possible candidate for secretary of treasury. Like his friend Bob Rubin before him, he would fit the profile of the Wall Street insider who knew how the economy worked from the trenches. But Obama demurred, picking Timothy Geithner instead. Jim, Steyer's loyal older brother, says Obama made a big mistake. "They should have listened to my brother," he says, "because he's smarter and richer than they are. It's that simple. Obama did not recognize Tom's talent. He will rue the day that he failed to recognize it."

Lacking traction with the new administration, Steyer decided to strike out on his own. In 2008, he was among a group of donors who gave $100 million to Stanford to build an energy research institute. He also began preparations for handing over the management of his investment company to a partner so he could clear the decks for a move into politics. In 2010, he co-chaired a campaign, along with then governor Arnold Schwarzenegger and former Reagan secretary of state George Shultz, spending $5 million to defeat a proposition that would have overturned California's aggressive greenhouse-gas-emission laws. He then underwrote a series of ballot initiatives in California, among them an initiative to close corporate tax loopholes and funnel the extra revenue into education and the environment. That one cost him $30 million, but he won again, emboldening him for an entry to the national stage.

As Steyer's stock rose among democratic pooh-bahs, he befriended former Clinton adviser John Podesta, who invited him to join the board of the Center for American Progress. During hiking and snowshoeing adventures together, Podesta encouraged him to start NextGen Climate Action Committee to take the fight to Washington and beyond. "He thinks about his kids, and he thinks about the future, and that's what got him into this," says Podesta.

All Steyer needed was a convincing message. And in July 2012, he found one in Bill McKibben's essay in Rolling Stone, which described the critical science behind climate change, painting a dark picture of impending doom. McKibben argued for an urgent new movement in which universities, churches, and governments divest from oil companies that foster climate change, with the Keystone XL pipeline as the front in an escalating war against fossil fuels. Set on fire by the story, Steyer called McKibben and asked him to go hiking in the Adirondacks in upstate New York, near McKibben's Vermont home.

"He proved, among other things, to be an astonishingly good hiker," says McKibben. "He's a logical guy, and logic would dictate that we could take action in the face of a serious crisis. I think he comes from a world of business where people do act on things. And if they don't, they go out of business. His sense is that we may well be going out of business."

Soon after, Steyer pledged to pull his personal money from environmentally unsound investments, something his wife says "he's free to do because he's no longer representing a bunch of investors." He's representing himself. (Steyer remains a limited partner at Farallon, but has segregated his own money into a separate fund that is free of ecologically unsound investments.)

Steyer now trained his lasers on the Keystone pipeline. As a former energy investor, he understood the vulnerability of the oil companies. If he could get President Obama to cancel the pipeline, he argued, the higher costs of distributing the Canadian oil using trucks and ships would make the oil sands less profitable and diminish the flood of fuel into the market. (To be sure, others disagree with this analysis, arguing that global demand will find a way to burn the fuel one way or another.)

The idea was to make Keystone an inflection point in the larger struggle to bring climate change into the center of the political conversation in America. "My experience of American politics is that people raise issues and they get addressed in an effective but imperfect way," says Steyer. "But that's sort of the American system: Mind the problem and worry it, and then we attack it with overwhelming power and put it away – and that's the end of that problem."

Tom Steyer, in a crisp blue oxford and a Royal Stewart tie, is sitting in a wood-paneled restaurant in Arlington, Virginia, now with two fresh-faced media handlers by his side, both in jackets and ties and feverishly punching out emails on BlackBerries. It's November 2013, the day after the Virginia race for governor, and Steyer is philosophical, mulling over what his money has bought him here. Democrat Terry McAuliffe has eked out a two-percentage-point win over Republican Ken Cuccinelli, a climate-change denier.

It wasn't clear whether Steyer's cash infusion in the race – $8 million all told – had been a crucial factor. But for him, winning the race was only half the point. He wanted to turn McAuliffe's opponent into a virtual billboard for public awareness, and he convinced himself that he'd achieved that goal. "Not acknowledging that there's an energy climate problem was one of the reasons that Cuccinelli was disqualified from being a governor of Virginia," he says, "because he's too out of touch. If Americans decide that if you won't accept basic science, you're too out of touch to hold a basic position – that's good. That's really important."

This was Steyer's second big intervention in a political race with his super PAC. His first strike was in Massachusetts last spring during the Senate race to replace John Kerry, then incoming secretary of state. Steyer made an immediate splash by flying a banner over Boston that read steve lynch for evil oil pipeline. Edward Markey won the race handily, and Steyer managed to successfully introduce himself as the new rich guy ready to rumble on climate.

"We made them talk about climate," he says. "We made the people of Massachusetts talk to each other and think about climate and think about what's responsible to do, and that's a huge change."

The Massachusetts and Virginia races were only a beta test for the upcoming midterm elections, says Steyer's adviser Chris Lehane – Steyer will inject millions into assorted races in a much more dangerous election cycle for the Democrats, when Republicans are threatening a wholesale takeover of Congress. But all the while, Steyer has stayed focused on the White House. After we met in San Francisco last spring, he threw a lavish fundraiser, where he personally urged President Obama to stop the pipeline. Obama listened respectfully but explained that halting it was difficult without convincing people that its economic benefits were overstated. Steyer saw this as an opening, explaining to Obama that talk of the pipeline creating 6,500 American jobs was a fantasy cooked up by the oil companies. Steyer says it will create only 35 jobs. "We were saying all along that these job numbers are fictitious," he recounts. "Here's what they actually believe, what they put down in black-and-white. Just take their numbers. And the president took those two points. We were talking about the economics of this."

Obama had too much political pressure from the right on health care and the budget to fight for climate just yet. But he was getting enough push from his political left, not least from Steyer, that he declared last summer that he was going to evaluate the Keystone project "based on whether or not this is going to significantly contribute to carbon in our atmosphere," adding that there was "no evidence" that the pipeline was a significant generator of jobs. Though many observers read Obama's statement as ambiguous, a stopgap to give him time to stave off the issue so he could fight other battles in the short term, Steyer saw a marked shift – and his own fingerprints.

"We talked about the impact of gasoline prices in the Midwest – which he quoted!" he hooted. "To me, it looked fairly straightforward, like he was giving himself space to say no. And that doesn't mean he's definitely going to say no. He was refuting the arguments as to why this was an important thing to say yes to. From my point of view? For him? This is an obvious decision."

With the politics getting increasingly hazardous for Obama, Steyer says the president will come to his senses on the pipeline because it's an easy way to claim a legacy by fiat, without a battle in Congress: He simply cancels the license for the Keystone XL pipeline and he's an automatic hero for history. In that sense, Obama and Steyer are in this one together. "Everyone thinks that Obamacare is going to be his legacy, and it will be in the short run," Steyer says. "But his legacy issue 20 years from now will be this. And a hundred years."

One positive sign did emerge after the Virginia race: In December, Steyer's friend John Podesta was hired by Obama as a counselor, putting an important Steyer ally and climate advocate within arm's length of the president and the Keystone decision.

Another positive: Steyer seems to be having fun. "It's unpleasant to target people with bad messages and make them miserable," says Taylor. "A lot of people wouldn't want to do that – personally, by professionals, with repercussions. But he's quite willing. He gets energy from combat."

Meanwhile, Steyer is teaming up with his old Goldman Sachs pals, Hank Paulson and Robert Rubin, along with Mayor Michael Bloomberg, to finance a study on the costs of inaction in climate change. Once the data is out there, Steyer figures, no candidate who wants to win political office will be able to ignore it. "You can't be president of the United States if you don't believe in gravity," he says. "It's just dumb. Too much stuff depends on us accepting data and using it to inform our decisions."

Logic and data and money are indeed powerful instruments. They were certainly the lifeblood of Tom Steyer's career. But the political path to success on climate-change activism, from the trenches of state races to the campaign to influence a president, is an incremental slog. By comparison, the golden ladder of an all-American overachiever, from Exeter to Yale to Stanford to Goldman and beyond, looks well-defined. And indeed, Steyer is not discounting a return to that ladder – in particular, the well-trod route of wealthy donors to positions of power in the White House. Already, his closest associates are speculating about what high office Steyer might be suitable for if, for instance, he is tapped by a future president Hillary Clinton in 2016. Treasury secretary? Secretary of energy?

Says his brother Hume, "If she does win, she might give him a job. And if she offered him a job, he might take it."

Barring that, he could run for office in California, where he has promised to pour more millions into a new campaign to tax all oil extracted in the Golden State. As he climbs into a minivan to be driven to the airport, where he'll fly commercial home to San Francisco from Washington, Steyer keeps the door wide open. "I'm not doing this to build my résumé," he says. "I'm doing something to get something done."

Then he flashes a smile that is worth a billion dollars. "If I could do something that would be helpful," he says, "I would do it."