These patterns are likely to continue: The current partisan geography is a natural political alignment. Around the world, urban areas tend to be left-leaning and cosmopolitan; rural and suburban areas tend to be conservative and populist.

This balance was thrown off for decades in the United States because the New Deal created an unlikely Democratic Party alliance between urban liberals and rural Southern conservatives (who were Democrats primarily because Republicans were the party of Lincoln and Reconstruction). This alignment could hold as long as civil rights was kept off the agenda. Yet, even after the turbulent 1960s, it took 50 years to cement the current arrangement.

One reason for the slow realignment in Congress was that in the 1970s and 1980s, many incumbent members of Congress mastered the art of constituency service, pork-barrel politics, personal self-promotion and enough fund-raising to scare off potential challengers. With weak party leadership in Congress, members were also free to “vote their districts,” even if that meant crossing party lines. This contributed to a blurring of partisan differences, which made individual candidates more important to voters than simple party labels. Partisan loyalties are also very, very sticky: Until 2010, Democrats held the majority in the Alabama State Legislature.

Perhaps the clearest measure of this slow transformation was the high rate of ticket-splitting that lasted for decades. From 1956 to 1996, on average 32 percent of House districts split their tickets, backing one party in the presidential race but the other for Congress. In 2012, that measure fell to 6 percent, the lowest since 1920 (3 percent). From 1968 to 1988, roughly half of states voted for different parties for the Senate and for president; in 2012, only six of 33 (18 percent) did.

As the parties became more homogeneous, rank-and-file members began to cede more authority to their leaders to enforce party discipline within Congress, especially in the House. Particularly after the watershed election of 1994, when many longtime conservative Democratic seats turned into relatively safe Republican seats, a new generation of conservative lawmakers and a newly assertive party leadership exerted a hard-right pull on the Republican Party. That election also bled the Democratic Party of many of its conservatives, shifting its caucus to the left. The election of 2010 was the culmination of the decades-long undoing of the New Deal coalition, sweeping away the few remaining Southern conservative Democrats.

Moreover, as more of the country became one-party territory, the opposing party in these places grasped the improbability of winning and so had little incentive to invest in mobilization and party building. This lack of investment further depleted a potential bench of future candidates and made future electoral competitions less and less likely.

These trends have been especially bad news for congressional Democrats, whose supporters are both more densely concentrated into urban areas (giving them fewer House seats) and less likely to vote in nonpresidential years (when most elections for governor are held, robbing the party of prominent state leaders). Since Republicans hold more relatively safe House seats, Democrats might benefit from occasional wave elections when the Republican brand has been significantly weakened (e.g., 2006 and 2008). But given the underlying dynamics, such elections are far more likely to be aberrations than long-lasting realignments.