The Cook Partisan Voter Index, first published in 1997, rates the degree to which a congressional district leans Republican or Democrat by comparing the district's voting history in presidential elections to the nation as a whole. The Cook PVI tracks the partisan ratings of all 435 congressional districts and all 50 states.
A swing district, according to Cook, is defined by a PVI rating ranging from D+5 to R+5.
For more than two decades, the PVI has helped map the trends of our country’s shifting political landscape, allowing organizations to contextualize the policy work they are doing with the politics of a legislator’s district. The release of the 2017 PVI ratings reveal powerful insights into the post-Trump electorate.
With the PVI ratings and the 2020 elections on the horizon, legislators in 29 states have introduced over 100 bills calling for changes to their state’s redistricting process or for the creation of an independent redistricting commission. In addition, organizations are working to build relationships with new candidates in districts considered “Toss Ups” by PVI ratings