Ideology scores are one of the most powerful tools for understanding Congress — and one of the most misread. Here's what they actually mean.
Find ideology data on any member dossier's Scorecard tab or at capitoltrace.com/polarization.
DW-NOMINATE (Dynamic Weighted NOMINAl Three-step Estimation) is a statistical model developed by political scientists Keith Poole and Howard Rosenthal at UCLA. It calculates ideology scores for every member of Congress based entirely on voting records — not self-reported party affiliation, not campaign rhetoric.
The model analyzes every roll call vote and places members on a liberal ↔ conservative scale from -1.0 (most liberal) to +1.0 (most conservative).
| Score range | Interpretation |
|---|---|
| -1.0 to -0.5 | Strongly liberal |
| -0.5 to -0.2 | Moderately liberal |
| -0.2 to +0.2 | Moderate / centrist |
| +0.2 to +0.5 | Moderately conservative |
| +0.5 to +1.0 | Strongly conservative |
The ideology gauge on every dossier shows where the member sits on this scale, how they compare to their party's median, and how their score has changed over time.
Separate from the ideology score, Capitol Trace calculates a bipartisan score: the percentage of votes where a member voted with the majority of the opposing party.
A high bipartisan score doesn't mean moderate — it means cross-aisle. A very conservative member might occasionally vote with Democrats on specific issues (veterans benefits, local infrastructure) while remaining ideologically conservative on most things.
The party unity score measures how often a member votes with their own party's majority. High party unity = reliable party vote. Low party unity = independent streak (or represents a district that leans the other way).