The Cook Political Report has introduced the 20th anniversary edition of the Partisan Voter Index (PVI) for all 50 states and 435 Congressional districts. Compiled by POLIDATA, the index has been updated to incorporate results from the November 2016 presidential election.
The PVI measures how each district performs at the presidential level relative to the national average. For example, a score of D+2 indicates that a district performed an average of two points more Democratic than the nation in the 2012 and 2016 presidential elections. A score of R+4 indicates a district performed four points more Republican than the national average. Districts performing within half a point of the national average in either direction are assigned an EVEN score.
Data from the 2016 election showed the Republican presidential nominee carried 230 districts, while Hillary Clinton carried 205. This resulted in House Republicans winning 49 percent of all votes to House Democrats' 48 percent, despite Republicans winning 47 more seats. The House has become more 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 districts, known as swing seats. In 1997, 164 districts were scored between D+5 and R+5. By 2017, that number fell to 72 districts, representing a 56 percent decline since 1997 and a 20 percent decline from four years prior.
According to the report, natural geographic sorting of the electorate has driven more polarization than redistricting. Of the 92 swing seats that have vanished since 1997, 83 percent resulted from natural geographic sorting, while 17 percent resulted from changes to district boundaries. Additionally, the gap between the median Democratic-held seat and the median Republican-held seat has grown from 14 points in 1997 to 25 points today.
The PVI serves as a tool to isolate and quantify the impacts of sorting and redistricting on the makeup of House districts. The Cook Political Report previously released updated PVI scores in odd-numbered years following presidential elections and in even-numbered years following redistricting. The next update is scheduled for 2021 to reflect the results of the 2020 presidential election.
