Cook Partisan Voting Index


The Cook Partisan Voting Index, abbreviated PVI or CPVI, is a measurement of the partisanship of a U.S. congressional district or U.S. state. This partisanship is indicated as lean towards either the Republican Party or the Democratic Party, compared to the nation as a whole, based on how that district or state voted in the previous two presidential elections.

History

The Partisan Voting Index was developed in 1997 by Charlie Cook of The Cook Political Report, in conjunction with Clark Bensen and his political statistical analysis firm, Polidata, "as a means of providing a more accurate picture of the competitiveness". It is based on the methodology introduced by Rob Richie of the Center for Voting and Democracy for the Center's July 1997 Monopoly Politics report.
The Cook Political Report has since released new PVI scores every two years. In 2021, the newsletter ended its relationship with Polidata and instead used Dave Leip's Atlas of U.S. Presidential Elections to calculate 2020's results. The most recent iteration is the 2025 Cook Partisan Voting Index.

Calculation and format

The index looks at how every congressional district voted in the past two presidential elections combined and compares it to the national average. The Cook PVI is displayed as a letter, a plus sign, and a number, with the letter indicating the party that outperformed in the district and the number showing how many percentage points above the national average it received. In 2022, the formula was updated to weigh the most recent presidential election more heavily than the prior election.

By congressional district

The PVIs for congressional districts are calculated based on the 2020 and 2024 presidential elections. With a PVI of R+1, was determined to be the median congressional district, meaning that exactly 217 districts are more Democratic and exactly 217 are more Republican than this district., 219 districts are more Republican than the national average, 207 districts are more Democratic than the national average, and nine districts match the national average. The number of swing seats, defined as those between D+5 and R+5, is 97.
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