LIKE THE BEST AMERICAN SAGAS, this one begins on the Mississippi River.

David King Udall was born in St. Louis in 1851 to English immigrants venturing upriver from New Orleans. Mormon converts, they had been called to forge a holy civilization in the West, so they crossed the ungrazed grasses of Nebraska in wagons pulled by oxen. "The wilderness and the solitary place shall be glad for them," said the Book of Isaiah, "and the desert shall rejoice, and blossom as a rose."

Baby David made the journey in swaddling clothes. The child who would one day sire a political dynasty peered out from his bonnet as the wagon train climbed the jagged Rockies and descended into Brigham Young's settlement on the Great Salt Lake. He would indeed make the desert blossom, ingrained as he was with a belief that the land's bounty was to be nurtured and shared. As a teenager, David caught a man drawing water from the town canal out of turn. "He and I had words and finally came to a hand-to-hand tussle," he wrote later, "in which it happened that I, being a husky youngster, threw the fellow into a deep hole in the ditch."

In 1880, at the church's behest, Udall and his family traveled overland to help settle Arizona, where he was elected to the territorial legislature, starting a tradition of stewardship and public service that would become a Udall trademark. His descendants have included four U.S. representatives, a U.S. senator, a secretary of the Interior, two chief justices of the Arizona Supreme Court, a handful of mayors and judges, and a candidate for president.

What has distinguished the family most is conservation, a legacy built by two of David's grandsons, brothers Stewart Udall and Morris "Mo" Udall, who stepped onto the national stage in the early 1960s just as the modern environmental movement was taking shape. As secretary of the Interior under John F. Kennedy and Lyndon Johnson, Stewart helped pass America's most visionary conservation laws, including the Endangered Species Act, the Wilderness Act, the Clean Air Act, and the Wild and Scenic Rivers Act.

Mo, a 30-year member of Congress from Arizona and longtime chair of the House Committee on Interior and Insular Affairs, was the force behind the nation's single most sweeping preservation law: the 1980 Alaska National Interest Lands Conservation Act, which added more than 56 million acres of wilderness, 54 million acres of refuges, and ten new national parks. Congress has dubbed both America's easternmost point, in the U.S. Virgin Islands, and its westernmost, in Guam, Point Udall—for Stewart and Mo, respectively.

In recent decades, however, a lot of the Udalls' work has met with a backlash,one led by oil, gas, and coal industries and anti-regulatory western senators like Pete Domenici of New Mexico, Ted Stevens of Alaska, and Orrin Hatch of Utah. President George W. Bush has aligned himself with this cause, overturning the Roadless Area Conservation Rule and appointing coal and timber lobbyists to high posts in the departments of Interior and Agriculture. The pendulum swung farthest in 2003, when Richard Pombo, a California Republican who wanted to gut the Endangered Species Act and drill in the Arctic National Wildlife Refuge,became chairman of the House Committee on Natural Resources, Mo's old post.

These days the momentum is shifting again. Our binge on natural resources has left us with a bad case of morning-after shakes. Pump prices have spiked. The war in Iraq illustrates every day the cost of our dependence on Middle Eastern oil. Hurricane Katrina provided a grisly glimpse of the worst-case projections about climate change. With Al Gore winning the Nobel Peace Prize and all the major presidential candidates sounding greener than the incumbent, conservation is returning to the mainstream.

Yet the one thing this revitalized movement still lacks is strong leadership in Washington. While the battles are not strictly fought along party lines—Republicans Susan Collins and Olympia Snowe of Maine often vote green, while Louisiana Democrat Mary Landrieu frequently sides with oil and gas interests—the role of western Republicans has been consistent. With the notable exception of Arizona's John McCain, they have marshaled their power against conservation. And although Alaska and the states of the Intermountain West—Idaho, Montana, Wyoming, Nevada, Utah, Arizona, Colorado, and New Mexico—are home to just 7 percent of the country's population, their senators control 18 percent of the chamber, giving them broad power to derail legislation.

As Randy Udall—Mo's 56-year-old son and director emeritus of the Community Office for Resource Efficiency (CORE), an energy nonprofit in Colorado—puts it: "The future of climate policy will be determined by western senators."

Which is where the Udalls come in—again. In 1998, Mo's son Mark, from Colorado, and Stewart's son Tom, from New Mexico, were elected to the House. This year both men are running for Senate, and if they win, their victories might decisively tip the balance of power. For the first time in a generation, the Udalls could be the ones tossing bad guys into the ditch.