CAPITOL HILL WAS atwitter. It was January 4, 2007, the first hundred hours of the 110th Congress, and Democrats were taking the reins after 12 years of Republican rule. Among other things, the Dems were promising to pass the most significant energy bill in memory, one that would end oil and gas subsidies, limit oil leases in the Gulf of Mexico, and promote investment in renewable energy sources.

I was stomping down the marble corridors of the Cannon House Office Building, hurrying to keep up with Mark Udall. A rangy six foot one, Mark wore polished cowboy boots and pleated slacks secured with a tooled silver buckle. His graying hair was disheveled and his eyebrows bushy—by Hill standards, he looked positively windblown. Now 57, he's the only member of Congress to have attempted Mount Everest, getting turned back 3,000 feet below the summit by storms. He did, however, climb the world's third-highest peak, 28,169-foot Kanchenjunga, in 1990. And he summited solo.

Mark's father died in 1998, but Mo lives on in his son's office. A campaign poster from his 1976 presidential bid dominates a wall, and up on a shelf sits a pair of his size 16 basketball sneakers—six-foot-five Mo, a star at the University of Arizona, played a season for the Denver Nuggets.

When Mark was elected, western Democrats were an endangered species in the House, holding just three of the 25 seats in the Intermountain West. "We formed the Coyote Caucus," he told me. "Sometimes it was just my cousin Tom and I having a beer." But Democrats have surged in the region, picking up nine more House seats, two Senate seats, and five governorships. They've positioned themselves as conservationists, forging alliances with sportsmen and farmers while painting Republicans as tools for mining and gas.

Mark's buzzer vibrated and I followed him at a trot into a system of underground tunnels beneath the Capitol. We leaped into an elevator, where we ran into Tom.

"Marcus!" cried Tom, slapping Mark on the shoulder.

"Tomás!" said Mark.

The two have a knack for chance meetings. In 1989, Mark was descending from the summit of Argentina's 22,834-foot Aconcagua when he encountered another group of Americans. "I see this guy coming up towards me and think, That guy looks familiar," Mark told me. "It was my cousin Tom!"

Tom, who has also climbed 20,320-foot Mount McKinley, the highest point in North America, had on a plain dark suit with cowboy boots. At 59, with slightly stooped shoulders and thinning hair, he looks like an earnest country lawyer. Tom has been a federal prosecutor and New Mexico's attorney general. Having once watched him grill the chief of the Bureau of Land Management at a congressional budget hearing, I had seen firsthand his encyclopedic recall of environmental law.

During their years in the minority, the Udall cousins had managed a few small victories: Mark helped win wilderness designation for Colorado's James Peak and, with Republican cosponsors, the Spanish Peaks, and in 2005 Tom wrote a successful bill to protect New Mexico's spectacular Valle Vidal from mineral leasing. They'd also introduced a few ambitious bills that went nowhere, including the Renewable Electricity Standard (RES) Act, which would have required power companies to generate 20 percent of their juice from sources like wind and solar. They cosponsored the Peak Oil Resolution, which would have prioritized the search for alternatives to the world's dwindling oil supply, and they brought up a bill forcing mining companies to clean up abandoned sites. None made it out of committee.

Now, with a sympathetic Congress, the Udalls saw a chance to gain traction. In their first eight days of business, the House passed every part of the Democrats' "100 Hour" plan, including the energy bill. But each of those bills would be blocked by the Senate. One January morning I found Mark in the basement deli grabbing a plastic-encased snack.

"Sometimes I think this is a colossal waste of time," he told me, swiping his pay card. "I mean, ten years ago I was on Mount Everest. Things move so slowly here. It's hard to get anything done. But to be honest—and I don't mean this to sound too self-important—I think we need more people here with values like mine."