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.