The 2021 Cook Partisan Voter Index

The Cook Political Report introduced the post-2020 edition of the Partisan Voter Index for all 50 states and 435 congressional districts.

Here’s how it works: “A Partisan Voter Index score of D+2, for example, means that in the 2016 and 2020 presidential elections, that district performed an average of two points more Democratic than the nation did as a whole, while an R+4 means the district performed four points more Republican.”

Last year saw a slight increase in the number of swing districts as former Republican suburban seats moved left and midwestern Democratic seats moved right.