If anyone in Kootenai County could have predicted the Democrats' downfall, it was Dan English. He had spent most of his life in the Idaho Panhandle and monitored more than 100 local elections in his 15 years as county clerk. The first ballots he counted, in 1996, revealed tight contests between Republicans and Democrats, but in the years that followed, the margins only widened. By 2002, the Democratic presence had been so whittled down that only one Democrat -- English himself -- still held an elected county office. For his re-election campaign that year, he distributed wooden nickels labeled, "Save the Last One," reminding voters of a bygone time when his party dominated the county. That caught the attention of USA Today, which observed that English was a rare political survivor in what had become "the most Republican county in the most Republican state in the nation." Once again, English was spared.

But by Nov. 2, 2010, when he faced another election, Kootenai County had swung even further to the right. President Obama was especially unpopular with Idaho Republicans, and any association with his party and policies had become a political liability. English is a gentle, affable man with bipartisan appeal: His children served on active duty in Iraq; he founded the nonprofit North Idaho Youth for Christ; and he was civically engaged well before he became clerk, serving on the school board and city council. English knew, however, that his record no longer mattered as much as the letter "D" beside his name. "You don't have anything to worry about. People like you," his friends assured him, but English had doubts. That November evening, he noticed the election supervisor studying the absentee ballots -- often a preview of the final totals -- with particular intensity. "I have to run this again. Something's not right," she told him. When she left the room, English pulled the results from the trash. "Sure enough, there I was, losing." He called his wife and said, "I think this may be the end of the run."

In the end, not a single Democrat was elected to a partisan office in Kootenai County. All three county commissioners, as well as the clerk, the assessor, the sheriff, the treasurer, the county attorney, and the coroner were Republican; so were the nine state legislators representing the area. Voters even backed a Republican candidate for the U.S. House of Representatives, Raúl Labrador, by a 10 percent margin over Democratic incumbent Walt Minnick. (Labrador is now one of Congress' most conservative members.)

To outside observers, it may have appeared that the county swung along with the nation's political pendulum. American voters leaned right in 2010, awarding Republicans a majority in the U.S. House of Representatives. But in Kootenai County, something far more enduring than partisan realignment had tipped the scales. As English put it, the 2010 election marked "the end of an era" -- not only politically, but demographically. Conservative newcomers, primarily from Southern California, had helped quadruple the county population since 1970. Allied with conservative North Idahoans, they systematically transformed the local politics.

It was part of a much larger pattern: Increasingly mobile Americans were deliberately seeking out communities that reinforced their own social and political values. Elsewhere, conservative emigrants helped push certain suburbs of Boise, Denver, Colorado Springs, Salt Lake City and Phoenix further to the right, while liberals relocated to urban centers and college towns. The shift had a polarizing effect: In 1976, less than a quarter of Americans lived in counties that voted overwhelmingly -- by more than a 20 percent margin -- for either presidential candidate. By 2004, nearly half of Americans did.

The consequences have only begun to emerge. Journalist Bill Bishop and sociologist Robert G. Cushing, in their widely praised 2008 book, The Big Sort, suggest that the U.S. has become a patchwork of ideologically distinct communities that elect representatives who are frequently unwilling to compromise. No wonder, they write, that Congress is gridlocked, and issues such as health care, which once crossed party lines, are now definitively partisan. "What happened," writes Bishop, "wasn't a simple increase in political partisanship, but a more fundamental kind of self-perpetuating, self-reinforcing social division." Americans had created communities that functioned as "social-resonators" in which they could hear the "amplified sound of their own voices and beliefs."

Indeed, Kootenai County's transformation suggests that the most indelible impacts may be felt in the echo chambers themselves -- in the counties, red and blue, where the majorities' values are reinforced in every facet of local government, and where it's easy to forget the way the other half thinks. "It's taking us a step back," one self-described conservative told me, "because by making our own private Idaho, we're insulating ourselves from the world."