Cook Partisan Voting Index

The Cook PVI is displayed as a letter, a plus sign, and a number. The letter (either a D for Democratic or an R for Republican) reflects the major party toward which the district (or state) leans. The number reflects the strength of that partisan preference in rounded percentage points. A district or state that "performed within half a point of the national average in either direction" is designated as "EVEN".[4]

PVIs are calculated by comparing the lead candidate's average share of the two-party presidential vote in the past two presidential elections to the party's national average two-party share for those elections. For the current calculation, the national average for 2016 and 2020 was 51.7% Democratic to 48.3% Republican.[3] Thus, the state of Montana, where Donald Trump won 61.1% of the two-party vote in 2016 and 58.4% of the two-party vote in 2020, for an average of 59.75%, has a PVI of R+11.[4]

Prior to its April 2009 update, the PVI formula compared district-level results for the past two presidential elections to nationwide results for only the most recent election. Since then, local elections are compared to synchronic national elections.[6]

House of Representatives districts map by PVI after the 2020 election

The PVIs for congressional districts are calculated based on the 2016 and 2020 presidential elections. The party representations are based on the results for the most recent election in that district for the 117th Congress. The District of Columbia's at-large congressional district, represented by a non-voting delegate, is ranked by Cook PVI, as it participates in presidential elections – its PVI is D+43.[4] Territorial districts are not ranked on the Cook PVI, as they do not participate in presidential elections.

As of 2021, in the House, there are 226 districts more Republican than the national average, and 203 districts more Democratic than the national average. Virginia's 2nd congressional district (R+1 PVI) was determined to be the median congressional district, meaning that exactly 217 districts are more Democratic and 217 are more Republican than this district. The number of swing seats, defined as those between D+5 and R+5, is 78.[4]

The PVIs for states are calculated based on the results of the U.S. presidential elections in 2016 and 2020.[4] The table below reflects the current state of Congress and Governors, based on the most recent election results, as of January 2022.

The most Democratic congressional district in the country is Pennsylvania's 3rd, located in West Philadelphia, with a PVI of D+41. The most Republican district is Alabama's 4th, based in North Alabama, at R+34. In terms of states as a whole, Wyoming is the most Republican, at R+26, and Hawaii and Vermont are the most Democratic, at D+15.[4]

In the Senate, the most Republican-leaning state to have a Democratic senator is West Virginia (R+23 PVI), represented by Joe Manchin. The least Democratic-leaning states to have two Democratic senators are Arizona (R+3 PVI), represented by Kyrsten Sinema and Mark Kelly, and Georgia (R+3 PVI), represented by Jon Ossoff and Raphael Warnock. The most Democratic-leaning state to have a Republican senator is Maine (D+1 PVI), represented by Susan Collins. The least Republican-leaning states to have two Republican senators are Florida (R+3 PVI), represented by Marco Rubio and Rick Scott, and North Carolina (R+3 PVI), represented by Richard Burr and Thom Tillis.

In the House, the most Democratic-leaning congressional district represented by a Republican is California's 21st (D+5 PVI), represented by David Valadao. The most Republican-leaning congressional district represented by a Democrat is Maine's 2nd (R+6 PVI), represented by Jared Golden. Following the 2020 elections, there were 28 Republican-leaning House districts represented by Democrats, and 7 Democratic-leaning House districts represented by Republicans.

• Political party strength in U.S. states
• Psephology, the statistical analysis of elections
• Two-party-preferred vote

• The Cook Political Report