What is the Cook Partisan Voting Index and how is it calculated for 2025?
Executive summary
The Cook Partisan Voting Index (Cook PVI) is a widely used metric that measures how strongly a U.S. congressional district or state leans Democratic or Republican relative to the national presidential vote; a label like D+2 means the area voted two points more Democratic than the nation in the measured presidential cycles (or R+4 for four points more Republican) [1] [2]. The 2025 Cook PVI was produced by comparing presidential two‑party vote shares from the 2020 and 2024 elections — with district-level 2020 results recalculated to reflect new post‑2020 boundaries using precinct data tools — and then referencing those averages against the national average to generate the D+/R+ score [1] [3] [2].
1. What the Cook PVI is and why political pros use it
The Cook PVI is a relative index, not a forecast: it captures the underlying partisan baseline of a district or state by showing how that place performed at the presidential level compared with the nation as a whole, which helps campaign strategists, reporters and analysts assess structural advantage and competitiveness beyond any single election’s noise [1] [4]. Originating in 1997 from Charlie Cook’s work and updated routinely by The Cook Political Report, the metric prioritizes presidential votes because they are the cleanest cross‑state comparison point for partisan lean [4] [3].
2. The step‑by‑step calculation used for the 2025 PVI
For the 2025 release, Cook used results from the 2020 and 2024 presidential elections: for each district or state, Cook averaged the Democratic and Republican two‑party presidential vote shares across those two cycles, subtracted the corresponding national average two‑party shares, and reported the difference as the PVI (e.g., D+2 or R+4) [1] [2]. Because redistricting after 2020 changed many district lines, Cook recalculated 2020 totals at the precinct level for each new district using Dave’s Redistricting App and VEST‑compiled precinct data to ensure the 2020 baseline aligned with current boundaries [3]. It is worth noting that Cook has tweaked methodology before — switching in the 2022 update to a 75/25 weighting favoring the more recent presidential result — and public summaries of the 2025 methodology emphasize the two elections but do not always make explicit whether a simple 50/50 average or a weighted average was applied in every instance, so methodological nuance can matter to analysts comparing releases [5] [3].
3. Technical choices and their implications
Relying on presidential two‑party vote share allows consistent cross‑district comparison but also imposes limitations: PVI measures presidential partisanship, not midterm or local voting behavior, and can miss ticket‑splitting, incumbency effects or turnout shifts that change House outcomes [4]. The need to reallocate 2020 precinct results into post‑redistricting maps introduces data‑sourcing complexity — Cook used Dave’s Redistricting App and VEST precinct data to do that work for 2025 — meaning small differences in precinct allocations or data compilation can nudge a district’s score by a point or two [3]. The Cook PVI is also proprietary content of The Cook Political Report and intended for internal analytical use subject to copyright — analysts often cite it but cannot republish the entire dataset without permission [6].
4. What the 2025 PVI release showed and the caveats for interpretation
Cook’s 2025 analysis highlighted a modest decline in geographic polarization even as national partisan divides remain deep, a finding driven by the aggregated district‑level shifts shown in the new PVI list and maps [7] [3]. By count, the distribution of districts in 2025 showed more districts tilted Republican than the national average and more tilted Democratic than the national average with a small set matching the nation exactly — specific tallies in public summaries note, for example, 219 districts more Republican, 207 more Democratic, and nine matching the national average in the 2025 characterization [8]. Readers and practitioners should treat PVI as a structural indicator — indispensable for baseline context — but not a crystal ball; it explains the terrain, not who will win any given race on a given day [1] [4].