The lodgepole pines are beautiful in Winter Park this year. Beautiful because they're dying. As you drop down from Berthoud Pass and roll past the famed old Colorado ski area, the mountainsides are carpeted with autumnal red and gold. To my Easterner's eye, they look like they'd make really neat Christmas trees. In fact, what they are is potential kindling.

The trees are that lovely rust color thanks to the mountain pine-bark beetle, a nasty little insect that looks like a stealth bomber shrunk down to the size of your pinkie nail. The beetles have infested this part of Colorado, as well as forested areas throughout the mountain West from Mammoth to Missoula. They basically attack each lodgepole and shut down its circulatory system, whereupon the tree turns from healthy green to desiccated red. In the past, freezing winters kept the beetles in check (a week of 20-below will kill them), but the winters are getting warmer now, and the bugs are taking over. Lower down the valley, the hillsides are covered with a mind-boggling number of doomed trees: the region's first victims of climate change.

On this clear, cold Saturday in late January, Colorado congressman Mark Udall and I speed past Winter Park toward a town hall meeting in the ranching and logging community of Granby. At 58, he's tall and lean, with an angular, handsome face and a thick shock of gray hair that could easily get him a job as a spokesmodel for Centrum Silver vitamins. As his campaign manager, Mike Melanson, drives, Udall is giving me a crash course on the biology of the pine-bark beetle. "They're beautifully engineered by Mother Nature," he says of the devastating vermin. "But Mother Nature has always had checks and balances in place."

About a half-hour drive down the road from Winter Park, Granby is ground zero for decaying lodgepoles – and approximately a political light-year away from Boulder, the geographical heart of Udall's congressional district. Today Udall will be chairing a summit meeting of first responders, local officials, state legislators, and representatives of various federal agencies. The topic of the day: what to do if, or rather when, those dying lodgepoles catch fire. A local rancher in full camo coveralls and a screaming-eagle cap corners Udall. "This is serious," he says. "A catastrophic fire would make Katrina look like a picnic."

The West just had one of the worst fire seasons on record, and, like sequels in a horror film franchise, they keep getting worse. Everyone in the room is tense. They know their asses are on the line, which is one reason FEMA is here in force, ahead of the disaster for a change. ("We like to think that to know us is to love us," the regional acting director says hopefully.) One carelessly tossed match could spell the end of thousands of acres of forest, major ski areas, tens of millions of dollars in mountainside homes – and Udall's hopes for winning election to the Senate in November. One of the lessons of Katrina, after all, was that failing to prepare for disaster can do a number on a politician's negatives. If Udall is perceived as letting his own district burn, no way are Coloradans going to give him a promotion.

After 10 years in Congress, Udall hopes to win the seat being vacated by the retiring Wayne Allard, a dowdy moral crusader whom 'Time' in 2006 named one of Washington's five worst senators. They couldn't be more different: Udall is defined by his strong record of conservation; he has a 95 rating from the League of Conservation Voters, or LCV, which carefully tracks the voting records of elected officials. This year, both the National Republican Senatorial Committee and the Democratic Party have declared that the contest between Udall and his Republican opponent, Bob Schaffer, a former congressman and an oil executive, is the nation's most important Senate race.

It's not just that the race is close, or that Colorado is a key swing state; Udall-Schaffer is a watershed moment for environmentalism's priority with voters in the West, where such issues – from water to wildfires to oil and gas drilling to exurban sprawl – are anything but abstract. A victory by Udall could be a stake through the heart of the big business – social conservative alliance that drills at will and has been the dominant force in the region for more than a dozen years.

Udall's opponent has tried to make "BoulderLiberal" into Udall's first name; the National Republican Senatorial Committee even operates boulderliberalmarkudall.com. Udall's last name, however, is more daunting to political foes. The Udalls are like the Kennedys of the canyonlands, minus the rum-running and sexual peccadilloes. A generation ago, Mark Udall's father, Morris – "Mo" – was one of the most powerful and well-liked members of Congress, while his uncle Stewart Udall served as secretary of the Interior under JFK and LBJ. Working together, they preserved millions of acres of wildlands while helping to pass the cornerstones of environmental protection in this country: the Clean Air Act, Clean Water Act, and Wilderness Act. Most famously, they first supported, then stopped, a dam project proposed for the Grand Canyon, which is the destination of the snow covering the mountains all around us here.

With a Democrat in the White House, Udall and his cousin Tom (a son of Stewart and a fellow congressman who is favored to win a Senate seat in New Mexico) could take office in a climate similar to that of the early 1960s, when environmentalism was fresh and there was a sense that anything was possible. "My father and my uncle are icons," says Udall. "But they also brought a pragmatic approach to these moments, when you could craft a historic new policy. I think the potential here is just huge."

As congressmen, both Mark and Tom Udall have taken up their fathers' mantle, championing renewable energy and conservation. As a direct result of Mark's work, Colorado is now a leader on alternative energy, with a multibillion-dollar wind- and solar-power industry. All of which makes it terribly ironic that, if those lodgepoles do go up in smoke, global warming could screw him out of a job.

Climbing up into the first creek drainage, just below Berthoud Pass, it quickly becomes clear that we're on the wrong path; or, if not the wrong path, then maybe not the best path. After the town hall meeting down in Granby, Udall shucked his western-congressman formal attire (sharp navy blazer, jeans, cowboy boots, turquoise belt buckle) for his favorite blue Marmot parka, and we burned rubber to get back up into the mountains in the hope that we might wring a decent backcountry ski run out of what's left of this brief winter day. We're on long nordic skis now, Udall, Melanson, and I, but the path winds and twists so tightly and steeply into the trees that we slide backward and get tangled up and topple into the two-day-old snow. That's what I'm doing anyway, cursing the fools who broke the trail.

"I'm guessing those guys were on snowshoes, for snowboarding," Udall says, barely breathing hard.

Even four years ago it would have been surprising that someone of Udall's progressive leanings could contend for statewide office in Colorado, a state that voted twice for George W. Bush and where Democrats rank third in voter registration behind Republicans and unaffiliated voters. His district straddles the Continental Divide, uniting wildly disparate elements as only certain congressional districts do. He's got Boulder, with its vegan triathlete Buddhists, as well as fleecy Winter Park and much of the I-70 ski corridor (including Vail), plus conservative ranching areas like Granby. The bulk of his constituents, however, live not in Boulder or Granby but in the blue-collar Denver suburb of Westminster. The Second District is "really a microcosm of the state," Lawrence Pacheco, Udall's former press secretary, tells me later.

Udall doesn't actually live in Boulder but in nearby Eldorado Springs (in part because it's close to his favorite climbing routes), and he's not known as a hard-line ideologue. On the other hand, he has supported Dennis Kucinich's legislation to create a "Department of Peace and Nonviolence." Whether the liberal label sticks is almost irrelevant at this point. The mere fact that a Udall – make that two Udalls – might be elected to the U.S. Senate in 2008 says a lot about the changes in the West since their parents retired from politics, particularly in Colorado.

"There's a fundamental shift going on," says Tony Massaro, political director of the LCV and a native Coloradan. It starts with the state's economy; mining and logging have been replaced by tourism and new-economy jobs, and telecommuters are flocking from other states so they can live near the mountains. The state legislature reverted to the Democrats in 2004, the same year the voters sent Democrat Ken Salazar to the Senate rather than the self-financed ultraconservative Pete Coors. Barack Obama could win the state for the Democrats at the presidential level, making only the second such victory since 1964 (Bill Clinton in 1992).

The Bush administration has unwittingly helped the Democratic cause by opening Colorado's Roan Plateau – a vast hunting and fishing paradise – to oil and gas leasing, over the objections of normally conservative sportsmen's groups who many believe will turn against Republicans in November.

"The policies of this current administration have created a pretty sour taste in the mouths even of folks who were ardent supporters of the administration four years ago," says Ken Neubecker, president of Colorado Trout Unlimited, a conservation group dedicated to the interests of freshwater anglers. "People see them for who they really are now."

Udall could be just the guy to profit from their discontent. He's carried his recent reelections with 35-point margins, thanks to a low-key, nonconfrontational style that disarms opponents. When conservative Granby and the surrounding ranching area was added to his district, he worked hard to bring the locals into his coalition. "I never burned any bridges, never made any fiery speeches, never made any accusations about being anti-environmental," he says.

"I don't know anybody who doesn't like Mark Udall," says Ben Nighthorse Campbell, a former senator who switched parties during his term.

When we pause to rest Udall points across the valley at 13,294-foot James Peak, which was somehow left unprotected in the patchwork of federal land holdings around Winter Park. Not long after he was elected to Congress in 1998, Udall set about trying to figure out how to fix that – no easy feat in a Republican Congress not prone to wilderness set-asides. The key, he realized, was getting locals onboard, as well as the rest of the Colorado delegation, which at the time was mostly Republican. "There were a lot of people who didn't want it to be wilderness," he says.

The solution was classic Udall. "Basically we've set it aside as wilderness, but we don't call it 'big-W' wilderness," he says. Instead, half of the 30,000-acre mountain is wilderness, while the other half is now known as the James Peak Protection Area, with certain areas still accessible to activities like mountain biking that are forbidden in "big-W" wilderness.

While the art of compromise can yield bipartisan successes like that of James Peak, one can't help but wonder if Udall isn't just too nice to survive a post–Swift Boat election, especially one so important to both sides. I'm reminded of the camo-clad rancher. I'd asked him if he was considering voting for Udall in November. He looked at me as if I were nuts, but the reason he gave was one I hardly expected: "He isn't aggressive enough."

The trail hasn't gotten any clearer, and the sun has almost disappeared. It's getting colder, but Udall wants to push higher. He's "whacked out from jet lag," he says, having just flown in from Afghanistan by way of a fundraiser in San Francisco, and we're facing a steep head wall, but his Extra Blue ski wax is working extra well. Ascending past a towering rock buttress, Udall scans it yearningly.

"Do you see any routes up there, Mark?" Melanson asks.

"I see a couple mixed routes, maybe," he says.

"Mike owes me a few days off," Udall grumbles as we continue to climb. He and Melanson keep climbing shoes in the car as they travel around the state, but Udall doesn't get out as much as he'd like to.

You're a weekend warrior now, I say, trying to kid.

He snorts. "I'm an hour warrior."

Mark Udall is the only person ever elected to Congress who's bagged an 8,000-meter peak, to say nothing of all 54 of Colorado's 14,000-foot mountains. The Udalls' affinity for the outdoors dates as far back as their penchant for politics. Mark grew up hiking and climbing in places like Zion Canyon, which his father and uncle had helped to preserve. But it was his mother, Pat, a Colorado native, "who got us outdoors and showed us the value of it, the meaning of it," Mark says. Later, as a student at Williams College in the 1970s, he convinced the school to let him spend a January term back home in Colorado, "studying" ski mountaineering.

After Udall graduated, he spent several months in 1975 working on his father's presidential campaign in New Hampshire before deciding that the family business was not for him. Instead he gravitated toward the mountains and the possibilities they offered, not least of which was the prospect of self-invention. "I didn't want to work in my father's shadow all of my life," he says. "I wanted to chart my own path through life, and I loved the outdoors."

Udall took a job guiding, first with an Arizona college's outdoor learning school and later with Outward Bound, living at the time with his brother Randy and sister Dodie in Crested Butte. Soon he met Maggie Fox, a young Outward Bound instructor from North Carolina; they married in 1982. "If you could have told me then that he would be doing this now," Maggie says today of the Senate race, "I would have laughed for two days."

In their off-seasons Mark and Maggie would climb, even topping a 25,000-footer in the USSR together before pregnancy and motherhood ended Maggie's mountaineering. In 1990, Mark was the only member of his team to summit Kanchenjunga, a beast of the Himalayas and the world's third-highest peak at 28,169 feet. On many other major trips, he climbed with a friend from Outward Bound, Tony Lewis, eight years his junior. They tackled Aconcagua, South America's highest peak, and in 1994 they attempted the forbidding North Face of Everest, reaching 25,500 feet before a fierce windstorm raked the mountain for four days and forced them back down. "Mark's been willing to hang it out climbing and hang it out politically," says Randy Udall, who has also climbed and skied with Mark. "He's clearly willing to roll the dice."

It was on Denali, not Everest, that the dice almost came up snake eyes. Udall and Lewis were above 18,000 feet when a storm blew in and trapped them on the mountain's face. "We'd brought seven days' worth of food," Udall says. "By the 10th day, we were getting pretty hungry." They didn't have enough rope and protection equipment to rappel down. "That's probably the most committed I've ever been," Udall says now. "It was either up and over the top, or perish."

"In a situation like that you really get to know the strength of someone's character," says Lewis, 50, who remains a close friend and confidant of Udall's. "People either stress out and freak out, or they can do it. That's why you go climbing with someone: You have to have total trust in them."

In the Udall family, politics stretches back generations. Mark and Tom's great-grandfather, David King Udall, was a pioneer Arizonan and a Mormon polygamist who was sent to federal prison (on testimony of an ancestor of Mitt Romney's, family lore has it). He was pardoned by President Grover Cleveland, then tossed back in jail. This time, it was Barry Goldwater's grandfather who got him off the hook. He eventually served in the Arizona Territorial Legislature, a legacy continued by his grandson Stewart, who served in Congress, and then by Mo, who was sent to Washington after Stewart joined JFK's administration in 1961.

"We grew up on the frontier, essentially," Stewart tells me by phone from Santa Fe. "We were close to the land. We milked the cows and worked on my father's farm. We learned conservation from the ground up."

After his father was appointed to Congress in 1961 (and before his parents divorced in the late 1960s), Mark would spend school years in Washington. "I remember like it was yesterday," he says today, "sitting in living rooms on the edge of those discussions between my father, my uncle, Bobby Kennedy, and [U.S. Supreme Court Justice William O.] Douglas. I was inspired by their vision, their belief that America could be even better than it was, and the sense that what they were doing was meaningful."

For 30 years, Mo Udall was one of the most admired members of Congress, wielding a quick, self-deprecating wit that disarmed foes. He was also among the most powerful lawmakers on Capitol Hill, chair of the Interior Committee, yet he gladly mentored younger members, including a fellow Arizonan named John McCain. McCain devotes a loving chapter of his book 'Worth the Fighting For' to "Mo."

Mark's father expressed no disappointment that his son hadn't taken up the family business; instead, he'd periodically send news clippings about the latest Himalayan tragedy without comment. "If he was unhappy with you, you couldn't really tell," Mark says. Udall achieved his own success, becoming executive director of Colorado Outward Bound, a job that allowed him to climb and lead expeditions. His father, meanwhile, had been diagnosed with Parkinson's disease, which forced him to retire in 1991 and soon left him incapacitated, leaving it to uncle Stewart to finally tell Mark, "You have some wonderful children; you've got to come down from the high country."

Mark left Outward Bound and fielded a few job offers that would have required relocating – including a pitch from Yvon Chouinard to come help run Patagonia, the outdoor clothing company – but he and Maggie decided they didn't want to leave Colorado. Driving around his district with him for a day in January, and again in August, I could see why.

One day he got a surprise call from local state rep Peggy Lamm, who said she would not be seeking reelection and invited Mark to run. Udall thought about it,and decided to roll the dice again. His campaign consisted of going door to door and shaking hands, sometimes with his uncle Stewart in tow, and he won a dark-horse race.

In his first year as a state representative, Udall introduced legislation that would define his career, the Renewable Electricity Standard. It would have mandated that 10 percent of the electricity used in Colorado come from alternative sources, such as wind and solar.

The bill died in his own committee. "I couldn't even pull the Democrats," Udall says now, shaking his head. "It was pretty bad." Within a year, the local congressional seat opened up, and despite the stinging defeat it seemed a logical move: There hadn't been a Udall in the House since his father retired.

In Tom DeLay's House and Dick Cheney's Senate, though, the signature Udall issue of environmentalism was a nonstarter. Where his father had never known a Republican majority, Mark was in the minority party until last year. Perhaps as a result, he's not afraid to break with Democrats; he voted for President Bush's "Healthy Forests" bill, which environmental groups described as an Orwellian prescription for just the opposite, and he has encouraged emergency logging of sick and dead lodgepoles as fast as they can be felled.

Udall has proved remarkably adept at working with Republican colleagues, the prime example being his continued efforts on renewable energy, a crusade he's never abandoned. In 2004, after three more defeats for the bill in the state legislature, Udall helped lead a statewide campaign for a ballot initiative, Amendment 37, requiring the state to produce 10 percent of its electricity from renewables. His co-chair was Colorado's Republican speaker of the house, Lola Spradley, a farmer's daughter representing a ranching district south of Pueblo; they were opposites in every way, but they traveled the state together stumping for the measure. In the end the voters approved Amendment 37, despite a relentless opposition campaign by the utilities and oil and gas companies.

Udall's energy standard has so far evaded passage in Congress, though not for lack of his trying: He and cousin Tom pushed it as an amendment to last year's energy bill, and it passed the House with 40 Republican votes, only to be stripped from the Senate bill with the acquiescence of Majority Leader Harry Reid. "Beyond disappointing," Udall said at the time. But with the likelihood of a stronger Democratic majority and a potentially like-minded president in office, he's more hopeful.

"We're biding our time, we're putting markers in place, we're proposing legislation – we're selling the ideas," he says.

"He's being modest," interrupts Melanson, Udall's political director. "His issue is now the debate of the nation."

As we descend into the first creek valley, the trail becomes unclear in the gathering darkness. We arrive at a slope of untouched snow and pause. Beyond it stands an impenetrable-looking wall of trees. "Go for it," Udall says. "You're young."

"You first," I say. He pushes off, making one good telemark turn before falling sideways into the powder. As he struggles to his feet, kicking and sloughing off snow, he yells, "This is not a metaphor for the campaign!"

Perhaps a better campaign metaphor comes one Saturday in late August in the form of a chilly downpour soaking Denver and neighboring Littleton, where Udall was supposed to spend the morning at a parade. The Denver suburbs are crucial to Colorado elections, but there will be no campaigning in Littleton today. The good news is that the rain (and snow, at higher elevations) is dampening the lodgepoles in the mountains, staving off the danger of wildfire for the time being.

He's wearing the same blue Marmot jacket he wore the last time I saw him, which was in January. "This is my Denali jacket," he says proudly. In the madness of the campaign, even his Gore-Tex threatened to become an issue, as opponent Bob Schaffer suggested that maybe it was a little too fancy, a little too yuppie, a little too Boulder for the rest of the state. The attacks faded when it turned out that the jacket was made by Coloradans right there in Colorado. But still. It's been a rough few months.

Back in June, barely eight weeks earlier, Udall had a comfortable 10-point lead in the polls, with momentum going his way. The LCV was running hard-hitting TV ads tarring Schaffer, who has worked for a Denver energy company since leaving Congress in 2003, as "Big Oil Bob." The group also named Schaffer to its "Dirty Dozen" list of anti-environmental legislators. And Schaffer had been further embarrassed when the 'Denver Post' published photographs of him parasailing in the Mariana Islands on a 1999 trip that was financed by jailed lobbyist Jack Abramoff. In July, his 19-year-old son Justin's Facebook page came to light, featuring the slogan "Slavery gets shit done."

But then gas prices jumped, and suddenly being in the oil business didn't seem like such a terrible thing for Schaffer; at least he might know where to get some. Udall found himself stumbling over campaign trip wires on even his signature issues, and the polls showed the Republican closing the gap, almost to a dead heat. In a debate, Schaffer challenged Udall to pledge to vote against adjourning Congress so the House could finish an energy bill. Udall foolishly accepted, and he raced back to D.C. He was sprinting up the Capitol steps when the vote was called, to the delight of Schaffer's campaign manager, Dick Wadhams, who vowed to "shove a bunch of 30-second ads up his ass." The guy behind the Swift Boat ads four years ago piled on, giving $400,000 to the ultra-right-wing Club for Growth for ads assailing Udall's tax record.

The attacks seemed to work: In mid-August Udall suddenly reversed his longstanding opposition to offshore oil drilling, a sacred tenet of environmentalism since the Santa Barbara offshore oil spill of 1969. So did Nancy Pelosi, Barack Obama, and other Democratic leaders, but it was Mark Udall whom the 'Wall Street Journal' singled out by name in a blood-in-the-water lead editorial challenging the Democrats to make oil drilling (somehow) a plank of their party platform.

It was a long way from the relatively genteel environment Udall's dad had known in the 1960s and 1970s and even 1980s, when a liberal Democrat could befriend a guy like John McCain even though they disagreed on many if not most issues. Whereas Mo lived amid Washington's social whirl, the life of the party, Mark spends weeknights in a 400-square-foot apartment on Capitol Hill, then flies home to spend weekends with Maggie and their kids, now both in college. Family rules stipulate no events on Sundays.

But today is Saturday, and Udall is headed for yet another face-to-face debate with the sharp-tongued Schaffer, his third in three days, this one at a Catholic church in Colorado Springs – evangelical country. Today the subject is immigration, and the bumper stickers in the parking lot seem none too friendly. (I didn't know the John Birch Society still existed.) The Springs is home to the U.S. Air Force Academy, and, predictably, Schaffer tries to body-slam Udall on the "Department of Peace" thing.

Afterward, as Udall wedges his lanky frame into the campaign Prius, he's amped on adrenaline from the debate. He has the air of a man who can't wait for November, one way or the other. He'll either be headed for the Senate with his cousin Tom, or he'll have a lot more time to go climbing and contemplate the connection I ask him about now – between climbing a mountain and another long, slow, frustrating process: politics.

"The difference," he says once he has calmed down, "is that when you're climbing a political mountain, there's another team up there trying to push you off."