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.

Polls commissioned by a partisan sponsor (a campaign, party, or aligned group, marked (D) or (R)) tend to lean toward whoever paid for them, so their margin is pulled back toward the center and they are given less weight than independent polls.

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).

A few races feature a credible independent, rather than a Democrat, as the main challenger (for example Dan Osborn in Nebraska). Because such a candidate consolidates the anti-incumbent vote and runs well ahead of a generic Democrat in a lopsided state, their prior adds an overperformance adjustment, widens the uncertainty, and ties them less tightly to the national partisan tide.

Montana is a three-way race: a Democrat and a credible independent are both on the ballot against one Republican. The seat still goes to whoever finishes first, so the model follows whichever challenger leads and treats the other as splitting the same pool of voters. That split carries a penalty, which is what lets a Republican win on a plurality even when the two challengers together outpoll them. Only the leading challenger carries a win probability; polls that offer just a head-to-head are set aside, because they describe a ballot that does not exist.

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 Census division (nine finer regions such as New England, the Mountain West, and the Deep South, rather than four broad ones), 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.

When a winning independent has said they would caucus with neither party, their seat counts toward neither majority. A strong enough independent can therefore leave the chamber hung, with neither party reaching a majority on its own — an outcome the forecast reports alongside each party’s chance of control.

Data & limitations

Live polling is compiled from Wikipedia’s race tables and 270toWin’s published poll listings (individual pollster results only), together with 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 and polling averages 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.