Politics 6 min read By Callum Montgomery
Model by data scientist Thomas Miller gives Republicans one-in-six chance to hold Senate
Thomas Miller's non-AI forecasting model, which draws on prediction market prices, puts the Republicans' chance of keeping the Senate at about 17 per cent five days before the midterm elections, down from roughly 35 per cent in mid-September.
Republicans have only about a one-in-six chance of retaining control of the Senate, according to a forecasting model built by Thomas Miller, a data scientist and professor at Northwestern University. As of around 9am on 30 October, five days before most polling stations close, the model gave the party a 17.1 per cent probability of winning the 50 seats needed to hold the upper chamber. On 17 September, the same figure stood at 34.8 per cent.
Miller's approach differs from conventional polling. He starts with the prices on two prediction markets, Polymarket and Kalshi, which he says trade the most money and where most of the betting activity takes place. He converts those prices into probabilities so that the odds for the two leading candidates in each race add up to 100 per cent. He then applies time series analysis to identify the underlying factors driving the betting prices and uses models drawn from finance and economics to estimate how those factors are likely to influence voters. Miller has said he believes his model performs better than AI-based approaches.
The model is updated every ten minutes and displayed on his website, Virtualtout.io, where the odds scroll across the screen in a band resembling a stock ticker. For each race, the site also shows the change over the previous 24 hours, a feature Miller calls the NowCast. He opened the site to public view about a week before the article was published, although he began running the numbers on 17 September.
According to Miller, one factor outweighs all others: Donald Trump. He argues that the states do not move independently of one another. If the odds rise for Democrats in one state, they are likely to rise across the board, and the reverse also holds. "The entire market is dominated by what Trump does and says hour by hour," Miller said. He estimates that his model accounts for 85 to 90 per cent of the variation in the Kalshi and Polymarket prices, and that his interpretation is that the movements are "overwhelmingly Trump and the Republican positions generally". When voters react negatively to the president's words or actions, Republican odds tend to fall.
The decline in Republican prospects has been marked over the past six weeks. In North Carolina, where Republican Thom Tillis is retiring and leaving an open seat, the party was never close and now has a 4 per cent chance of victory. In Texas, Republican odds have fallen from 46.15 per cent on 17 September to 31.9 per cent. In Ohio they have dropped from 45.4 per cent to 31.8 per cent, and in Alaska from 32.95 per cent to 28.6 per cent. Democrats have better than a 60 per cent chance in all five contests where they could flip a seat, and in most cases the figure is considerably higher.
Maine is the only state where Republicans are faring better than in mid-September. Susan Collins started as an underdog at 32.6 per cent but has climbed to 39.9 per cent, gaining 1.7 points in the previous 24 hours. In Alaska the Republican incumbent, Dan S. Sullivan, fell to a low of 23.4 per cent on 23 October before recovering to 28.6 per cent, an improvement of more than five points.
Political analysts looking for signs of serious trouble for the Republicans have pointed to traditional strongholds. In Iowa, Republican Ashley Hinton fell from 61.75 per cent on 17 September to a virtual tie at 53.4 per cent. In Kansas, Adam Hamilton was projected at 75.8 per cent six weeks ago and now stands at 57.1 per cent, a drop of almost 19 points. In the past day alone, Hinton and Hamilton lost 1.45 and 2.47 points respectively. In Michigan, where Republicans had hoped for a flip, their chances against Abdul El-Sayed have slipped from better than a third in mid-September to about 25.4 per cent.
If Democrats win every contest where their probability in the model is at least 60 per cent and hold the seats they already have, they would take a majority of 52 to 48, reversing the current 53 to 47 Republican advantage. In South Carolina, which Fox News recently moved from "lean Republican" to "tossup", the model still gives Darline Graham a 75 per cent chance, although her odds have slipped 2.4 points over the previous 24 hours.
Miller stresses that he is setting odds rather than making definitive predictions. A candidate at 30 per cent still has a reasonable chance of winning, and Republicans are above that level in Maine, Ohio and Texas, and close to it in Alaska. However, the party would need several upsets to hold the chamber while also defending seats it had not expected to have to protect. Miller's model suggests the contest is far from settled, but the trend has moved decisively against the Republicans in the final weeks of the campaign.
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