American politics has always been defined (or imprisoned) by Mason and Dixon, by regional rifts engendered by slavery, the Civil War and the Bible Belt. But now the relevant names are Lewis and Clark. The land west of the Mississippi, especially west of the 100th meridian, is the new swing region–and the place where Democrats hope to win the White House. “It’s our 21st-century-majority strategy,” said Simon Rosenberg of the New Democratic Network.

The 2006 election highlighted the old paradigm–and the new. The Northeast turned a deeper blue. GOP moderates there are all but extinct; Democratic governors were elected by vast margins. The Republicans’ Fortress South, up-armored with evangelicals by George W. Bush and Karl Rove, cracked at the perimeter but largely held. The “intermountain” West, meanwhile, was up for grabs. Democrats won the governorship in Colorado (and now have won five of the region’s eight statehouses since 2002). The GOP hung onto the governorship in Nevada and a key Senate seat in Arizona.

The westward trend is clear. For the first time, leaders of the House and Senate are from the Far West. Democrats put Nevada in a crucial early spot in the presidential-selection process, and will hold their convention in Denver. Republican front runners have Western roots (Sen. John McCain from Arizona, Massachusetts Gov. Mitt Romney, a leader of the Utah-based Mormons). Spanish-speaking Democrats (Gov. Bill Richardson of New Mexico, Sen. Chris Dodd of Connecticut) can go native.

As Hispanics grow in clout, so does the West, where most of the nation’s 43 million Latinos live. In 2004, 44 percent of their voters supported Bush; in 2006, only 30 percent did–driven away by the party’s get-tough rhetoric on immigration. The key presidential swing states now: the cluster of Arizona, New Mexico, Colorado and Nevada. The new map means politics could be less party-oriented, says Thomas F. Schaller, author of “Whistling Past Dixie.” Westerners aren’t “socialized to partisanship,” he says. And they are “libertarian” by nature, says Tester. The GOP must hold the faith-based while appealing to the less socially judgmental West; Democrats must avoid the big-government label, anathema to Westerners’ self-image. “We have an individualistic attitude,” Tester says. “It’s ‘Leave me alone and let me farm’.” That won’t be possible for Tester, however. He’s heading East, and turning over the farm to his daughter and son-in-law.