The national environment. Generic-ballot polls are averaged, weighted by recency and sample size, with pollsters who poll often weighted down so that no single firm dominates. Their crosstabs give each group's national level for 2026, and state polls that publish demographic breakouts are pooled in to firm those up. The national vote is what those groups add up to, not the raw poll average.
Who votes. Midterm electorates are older, whiter and better educated than presidential ones. Each group's turnout is projected from how it has held up in past midterms (Census CPS).
Candidates. An incumbent who outran or trailed the top of the ticket in 2024 carries that personal vote, but only while they are on the 2026 ballot. Seats with a single nominee, or two from the same party, are decided by the ballot rather than forecast. Senate and governor races also carry the incumbent's own last result and are blended with state polling where it exists.
Uncertainty. Each forecast is simulated tens of thousands of times. Win chances are the share of simulations each side wins, and the ratings on the map are drawn from those chances.