A forecasting model built by data scientist Thomas Miller now gives Democrats a 90% chance of winning at least 51 Senate seats, a far more commanding projection than most public polls and prediction markets currently show. As of around 9 a.m. Eastern on Monday, Miller's site, Virtualtout.io, put Republican chances of keeping control of the chamber under 10%, down from 23% on Oct. 26.
The sharpest shift is in Kansas, a longtime Republican stronghold where Democrats now hold the lead, and in Iowa, which the model has turned into a virtual tossup. In Kansas, Sen. Roger Marshall looked on track for easy re-election about two months ago at around 75%. His chances have since dipped to 43.9%, including a 7.1-point drop in the past 24 hours alone.
Miller, a professor at Northwestern University, previously drew attention for correctly forecasting that Democrats would sweep the two Georgia Senate runoffs in early 2021, when polls and pundits expected Republican wins in both. He also called Donald Trump's 2024 victory. His method starts with the prices posted on the two prediction markets with the highest trading volume, Kalshi and Polymarket, which he converts into probabilities for each party. He then applies multiple time series analyses that examine 10 to 12 underlying factors driving those markets and recalculates the forecast continually. The probabilities update every 10 minutes and stream across the top of the site in a band resembling a stock ticker, with a 24-hour change displayed alongside, which Miller says makes his figures far more current than polling.
Miller argues that the movement reflects voters' views of the Trump administration's performance. "They all relate to voters' perception of the performance of the Trump administration," he told Fortune. "That's the thread running through all the races. The factors include affordability, the economy, and the war in Iran, the issues that motivate voters. The states don't move independently of one another. When voters' view of those issues turns more negative, the Republican numbers drop on all the races." In his assessment, Trump has effectively nationalized the battle for the Senate.
Miller said the effect of Trump's campaign appearances is harder to measure. Most of what the president says on the trail, he said, reinforces views voters already hold. "Still, there's no evidence he's helping," Miller said. "It doesn't appear that his appearances are a positive." He cited one instance in which his model registered a clear backfire: after Trump praised an ad by Vivek Ramaswamy in Ohio featuring the mother of Democratic opponent Amy Acton criticizing her daughter, the Republican percentage dropped sharply.
Across the key races, Republican numbers have fallen substantially. In the five contests where Democrats have long hoped for a flip, Republicans stand below 30%, ranging from 28.4% in Alaska to 2% in North Carolina. Republicans' best remaining chance is in Maine, where incumbent Susan Collins sits at 34.7%. In Michigan, the probability of a Republican win over Abdul El-Sayed has faded from more than one-third in mid-September to 24.5%. In Iowa's open seat, Republican Josh Turek held nearly a 62% chance as of Sept. 17; he has dropped to 51.6% as Democrat Ashley Hinson pulls almost even.
Miller said that while many contests are moving only slightly from day to day, the cumulative gains toward Democrats over the last 10 days are substantial, and that the movement in states Republicans had counted on is surprising many observers. Virtualtout.io and its NowCast will run through Election Day and plan to call races first, based mainly on swings in betting odds. Republicans could still hold the chamber by keeping Kansas and Iowa, winning Maine, and defying the probabilities in Texas or another key state. That path, however, looks far narrower as Midwestern strongholds swing toward Democrats.