Cook Partisan Voting Index

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".^[5][6][7] 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.^[8][9]

The Cook Political Report has since released new PVI scores every two years.^[10] 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.^[7] The most recent iteration is the 2026 Cook Partisan Voting Index.^[11]

The index looks at how every congressional district voted in the past two presidential elections combined and compares it to the national average.^[12] The Cook PVI is displayed as a letter, a plus sign, and a number, with the letter (either a D for Democratic or an R for Republican) indicating the party that outperformed in the district and the number showing how many percentage points above the national average it received.^[10] In 2022, the formula was updated to weigh the most recent presidential election more heavily than the prior election.^[13]

The PVIs for congressional districts are calculated based on the 2020 and 2024 presidential elections.^[11] With a PVI of R+3, Pennsylvania's 10th congressional district^{[needs update]} 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.^[11] As of September 2026^[update], 224 districts are more Republican than the national average, 204 districts are more Democratic than the national average, and 7 districts match the national average. The number of swing seats, defined as those between D+5 and R+5,^{[citation needed]} is 89.^[11]

The PVIs for states are calculated based on the results of the U.S. presidential elections in 2020 and 2024.^[14] The table below reflects the state of Congress and governors, based on the 2024 election results.

Using the United States Census Bureau's official 2024 population estimates, a combined 182,737,780 residents live in the 29 states that lean Republican; 141,264,067 residents live in the 20 voting blocs (19 states plus Washington, D.C.) that lean Democratic; and 16,101,434 residents live in the two states (Michigan and Wisconsin) that lean neither Republican nor Democratic (PVI of EVEN).

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