The Cook Political Report has released its 2017 Partisan Voter Index (PVI), marking the 20th anniversary of the metric used to measure the partisan lean of all 50 states and 435 Congressional districts. The index, compiled by POLIDATA, incorporates results from the November 2016 presidential election to provide updated scores for the current congressional boundaries.
The PVI measures district performance at the presidential level relative to the national average. For example, a score of D+2 indicates a district performed an average of two points more Democratic than the nation during the 2012 and 2016 elections, while R+4 indicates a district performed four points more Republican. Districts performing within half a point of the national average are assigned an EVEN score.
Data from the 2016 election shows that the Republican presidential nominee carried 230 districts compared to Hillary Clinton's 205. This resulted in House Republicans winning 49 percent of all votes to House Democrats' 48 percent, despite winning 47 more seats. The House has become increasingly sorted, with only 35 of 435 districts crossing over to vote for presidential and House candidates of opposite parties, a decrease from 108 in 1996.
The report highlights a significant decline in competitive or swing seats. In 1997, 164 districts were scored between D+5 and R+5. By 2017, that number dropped to 72 districts, representing a 56 percent decline since 1997 and a 20 percent decline from 2012.
Research suggests that natural geographic sorting of the electorate has driven more polarization than redistricting. Of the 92 swing seats lost since 1997, 83 percent resulted from geographic sorting while 17 percent resulted from changes to district boundaries. Consequently, the gap between the median Democratic-held seat and the median Republican-held seat has grown from 14 points in 1997 to 25 points in 2017.
The Cook Political Report updates PVI scores following every election since 1996 and every round of redistricting since 2001. The next update is scheduled for 2021 to reflect the results of the 2020 presidential election.
