The most striking House statistic in the last 15 years may be the decline of competitive districts, places where members have the most incentives to work on a bipartisan basis. In 1998, our Partisan Voter Index scored 164 districts between D+5 and R+5, more than a third of the House, and greater than both the number of strongly Democratic and strongly Republican seats. After the 2012 election, there are only 90 districts between D+5 and R+5 – less than a quarter of the House and a 45 percent decline since 1998.
Redistricting is only responsible for a portion of the swing seat decline. In many minimally altered districts, the electorate has simply become much more homogenous. For example, the boundaries of WestVirginia's 2nd CD have barely changed since 1998, but its PVI score has shifted from EVEN to R+11 as its voters have moved away from the national Democratic brand. Likewise, Albuquerque's migration to the left has bumped the PVI score of New Mexico's 1st CD from R+1 to D+7.
But this voter self-sorting has also enabled partisan gerrymanderers to more easily polarize districts wherever they wield power over the map. As Robert Draper astutely observed in The Atlantic, the goal of partisan mapmakers is often to "design wombs" for your team and "tombs for the other guys." In the case of Northern Virginia's 11th CD, Republicans actually boosted Democratic Rep. Gerry Connolly’s PVI from D+2 to D+7 in order to make neighboring districts more Republican; Connolly's district is now D+10 following the 2012 election.