That’s from Charlie Cook’s column in the National Journal. He writes:
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.
In 1998, the median Democratic-held district had a PVI score of D+7, and the median Republican-held district had a PVI score of R+7—pretty partisan, but far from monolithic. Today, those median numbers are D+12 and R+10, and that 22-point gulf is the main structural driver of the political paralysis we lament today.