At National Journal, Charles Cook discusses changes in the Partisan Voter Index:
By now, the trend lines are clear. In 1998, we found 164 swing seats—districts within 5 points of the national partisan average, with scores between R+5 and D+5 (a score of R+5 means the district’s vote for the Republican presidential nominees was 5 percentage points above the national average). The data 15 years ago showed just 148 solidly Republican districts and 123 solidly Democratic seats. Today, only 90 swing seats remain—a 45 percent decline—while the number of solidly Republican districts has risen to 186 and the count of solidly Democratic districts is up to 159.
How did we get here as a country? Debates rage at political-science conferences: Are voters aligning in like-minded areas, or is blatant partisan gerrymandering to blame? Our newest index points mostly to the former, which has in turn amplified the power of the latter. In 2011 and 2012, redistricting diminished the number of swing seats from 103 to 99. But when we factored in the 2012 election results, the count fell more sharply, from 99 to 90.
In fact, setting aside redistricting, we found that 76 percent of Democratic-held House seats had grown even more Democratic in the past four years and 60 percent of GOP-held seats had grown even more Republican. Some districts swung dramatically, mostly along racial lines. Republican Rep. Hal Rogers’s 96 percent white district in eastern Kentucky jumped from R+16 to R+25, and Democratic Rep. Loretta Sanchez’s 82 percent nonwhite district in Orange County, Calif., moved from D+3 to D+9.
There’s no question this phenomenon has benefited Republicans. Observing Democrats trying to win a House majority today is akin to watching a soccer team play a comparably skilled opponent with the field slanted 25 degrees against them. Thanks to the concentrated nature of the Democratic vote, Republicans have always occupied at least two dozen more solidly partisan districts than Democrats. But the rise in ironclad districts forces Democrats to win a near-impossible share of what’s left just to even the score.