Methodology

How the 2026 Senate forecast is built.

The forecast combines what polls say with what the fundamentals of each race suggest, then runs tens of thousands of simulations to turn those estimates into probabilities. Every margin below uses the convention that a positive number favors the Democratic candidate.

1. Poll averages

Each poll is weighted by how recent it is (an exponential decay with a three-week half-life), its sample size, the population surveyed (likely voters count more than adults), and the pollster’s quality rating. A house-effect adjustment removes each pollster’s persistent lean before the average is taken.

2. Fundamentals

Before polls, each race has a prior expectation based on the state’s partisan lean, whether an incumbent is running, candidate quality, fundraising, and the national environment (the generic congressional ballot and presidential approval, with a midterm penalty for the party in the White House).

3. Blend

Polls and fundamentals are blended per race. Polls carry more weight as they accumulate and as election day approaches; a race with no polling relies entirely on its fundamentals.

4. Simulation

Forty thousand simulations each add three correlated errors to every race: a national error shared by all races, a regional error shared within a region, and an idiosyncratic per-race error (wider where we lean on fundamentals). Errors are drawn from a fat-tailed Student-t distribution. Counting the outcomes gives each race’s win probability, the distribution of Senate seats, the probability of a majority, and the tipping-point state.

Democratic control requires 51 seats because the Republican Vice President breaks 50–50 ties. Thirty-five seats are contested in 2026; the other sixty-five are held at their current party.

Data & limitations

Live polling comes from Wikipedia’s race tables and hand-entered overrides. Pollster ratings and historical polling come from the FiveThirtyEight data archive (CC BY 4.0); certified past results come from the MIT Election Data + Science Lab; fundraising comes from the Federal Election Commission. Third-party aggregator forecasts are used only for private cross-checking and are never republished. This is an independent project and is not affiliated with any of these organizations.