Overview

The Cook Partisan Voting Index (PVI) is a simple numerical measure used to describe how strongly a U.S. congressional district or state favors the Democratic or Republican Party compared with the nation as a whole. Presented as D+ or R+ followed by a number, the PVI indicates the partisan tilt of a geographic area based on recent presidential elections.

How it is calculated

Calculation compares the area’s average two‑party presidential vote share with the national two‑party average over the two most recent presidential elections. The difference is rounded and expressed as D+X or R+X. For example, if a district’s Democratic share averaged five percentage points higher than the national Democratic share, it would be labeled D+5. If the district matches the national average, it is often described as EVEN.

Uses and applications

PVI is widely used by political journalists, campaign operatives, and analysts to summarize partisan baseline conditions. It helps identify which seats are relatively safe for one party, which are competitive, and how redistricting or demographic change shifts the partisan landscape. It is a common input when allocating campaign resources and when forming election forecasts, though forecasters use it alongside other indicators.

Limitations and caveats

PVI reflects only presidential voting patterns and does not account for incumbency, candidate quality, local issues, turnout differences in nonpresidential years, or third‑party votes. It is a backward‑looking, structural measure rather than a precise prediction of any single contest. Values can change when boundaries are redrawn or after demographic shifts.

History and updates

The index was developed by the Cook Political Report as a standardized way to compare areas. It is updated regularly—typically after each presidential election and when new district boundaries are established—to reflect recent voting behavior.

Notable facts

  • PVI is available for both states and individual congressional districts.
  • Because it uses a two‑election average, it smooths short‑term variations but may lag behind rapid political change.
  • Analysts often pair PVI with polling, fundraising, and demographic data for a fuller picture.