Cook Partisan Voting Index

A map of House of Representatives districts by Cook Partisan Voting Index, as of August 8, 2018

PVIs are calculated by comparing a congressional district's average Democratic or Republican Party share of the two-party presidential vote in the past two presidential elections to the national average share for those elections. For example, the national average for 2004 and 2008 was 51.2% Democratic to 48.8% Republican.[4] In Alaska's at-large congressional district, the Republican candidate won 63% and 61% of the two-party share in the 2004 and 2008 presidential elections, respectively. Comparing the average of these two district results (62%) against the average national share (48.8%), this district voted 13.2 percentage points more Republican than the country as a whole, or R+13.[7]

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.[8]

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".[5]

The PVIs for congressional districts are calculated based on the 2012 and 2016 presidential elections.[9] The party representations are based on the current members of the 117th Congress. In the House, there are 241 districts more Republican than the national average, and 194 districts more Democratic than the national average.[5] The number of swing seats, defined as those between D+5 and R+5, is 72.[5]

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.[10] Territorial districts are not ranked on the Cook PVI, as they do not participate in presidential elections.

The PVIs for states are calculated based on the 2012 and 2016 presidential election.[10] As of February 2021, the table below reflects the current state of Congress.

*Includes an independent senator who caucuses with the Democrats.

The most Democratic congressional district in the country is New York's 15th, located in the Bronx, with a PVI of D+44.[5] The most Republican district is Texas's 13th, based in the Texas Panhandle, at R+33.[5] In terms of states as a whole, Wyoming is the most Republican at R+25, and Hawaii is the most Democratic at D+18.[10]

In the Senate, the most Republican-leaning state to have a Democratic senator is West Virginia (R+19 PVI), represented by Joe Manchin. The least Democratic-leaning states to have two Democratic senators are Arizona (R+5 PVI), represented by Kyrsten Sinema and Mark Kelly, and Georgia (R+5 PVI), represented by Jon Ossoff and Raphael Warnock. The most Democratic-leaning state to have a Republican senator is Maine (D+3 PVI), with Susan Collins. The least Republican-leaning state to have two Republican senators is Florida (R+2 PVI), represented by Marco Rubio and Rick Scott.

Four states with a Republican-leaning PVI (Kansas, Kentucky, Louisiana, and North Carolina) have Democratic governors, while three Democratic-leaning states (Maryland, Massachusetts, and Vermont) have Republican governors. These governors are all seen as centrists who attracted voters from the opposite party. The most Republican-leaning state with a Democratic governor is Kentucky (PVI R+15), with Andy Beshear, and the most Democratic-leaning state to have a Republican governor is Vermont (PVI D+15), with Phil Scott. Two of the three states with even PVI ratings, Pennsylvania and Wisconsin, have Democratic governors (Tom Wolf and Tony Evers, respectively).

In the House, the most Democratic-leaning congressional district represented by a Republican is Florida’s 26th; with a PVI of D+6, the district is represented by Carlos Gimenez, who is one of only six Republicans to represent a Democratic-leaning House district. The most Republican-leaning congressional district represented by a Democrat is Georgia’s 7th with a PVI of R+9, it is represented by Carolyn Bourdeaux. Following the 2020 elections, there were 26 Republican-leaning House districts represented by Democrats. The only Republican leaning state to have more Democratic House members than Republican House members is Arizona (PVI R+5) with 5 Democrats and 4 Republicans. The most Democratic leaning state to have more Republican House members than Democratic House members is Wisconsin (PVI EVEN), with 5 Republicans and 3 Democrats. The most Democratic district relative to its state is Tennessee's 9th, being D+28 in an R+14 state (a 42-point difference). The most Republican relative to its state is Illinois's 15th, being R+21 in a D+7 state (a 28-point difference). The most Republican states to have a Democratic Representative are Alabama, Kentucky, and Tennessee; while the most Democratic states to have a Republican Representative are California, Maryland, and New York. Of the 428 Congressional districts that are in states with more than one district, 104 lean to one party while their state leans to the other.

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

• The Cook Political Report