U.S. Senate races are increasingly decided by national forces rather than state-level politics or incumbency advantage. That makes election forecasts less certain than they appear, according to a new study in PS: Political Science & Politics by researchers at the University of California, Davis, and George Mason University.
Forecasters have the least historical evidence about the factor they most need to understand: how strongly national mood influences each race. National forces include presidential approval, partisanship and economic conditions, which can influence Senate races across multiple states at once.
The researchers' own forecast , which updates daily, is built on a new model that reflects the growing nationalization of congressional elections. It gives Democrats a 61.2% chance of winning control of the Senate in November and a 90% chance that Democrats will win between 46 and 56 seats. They need 51 to take control because the vice president breaks ties.
The model gives heavy weight to national conditions, but the data can't tell researchers how much weight to give them.
Christopher Hare , an associate professor of political science at UC Davis and co-author of the study, said races move partly on their own and partly with national tides.
"Because of that, we need to widen out our uncertainty intervals that much more," said Hare. "We need to say we're that much less confident about these races."
Why the past can't settle it
The study draws on eight Senate election years from 2010 to 2024, covering 269 races. That may sound like plenty, but all races in a given year face the same national political climate. Because that climate varies from year to year, researchers have only eight distinct election cycles to help them estimate how strongly national forces affect Senate races.
Same data, different odds
To see how much this matters, the researchers tested 120 plausible versions of their model. Each made different calls about how much incumbency helps, how much a state's political lean matters and how strongly national mood moves races. All but three fit past elections similarly well, yet they disagreed about who would control the Senate.
In the version analyzed in the paper, which reflects data as of Aug. 19, the chance of Democratic control ranged from 43.2% to 57.6%. About 89% of that uncertainty came down to one choice: how strongly national mood carries into Senate races.
What it means for pollsters
Hare said the study isn't a critique of pollsters. As fewer people answer their phones, forecasters have less data to work with and must account for the possibility that polls don't represent the full electorate.
"This is an ongoing and perhaps worsening source of uncertainty that we also need to account for in the models," Hare said. "Those 95% credible intervals assume random samples, and do we really have random samples? If not, then you need to expand that out."
What it means for voters
The authors recommend that forecasters show the range of possible odds, alongside the usual margin of error, and not present one number as if the data had settled every choice. Hare said that matters beyond statistics because forecasts shape how people think about whether their vote counts.
"There's something maybe a little bit disenfranchising about models that are too confident, which doesn't mean again that we should say that all models are 50-50 just to get people to go out and vote," Hare said. "But it's really dangerous if we have bad models that are overstating their confidence and that's depressing turnout when the race is potentially a lot closer."
Hare compared election forecasts to weather forecasts. Acknowledging uncertainty isn't the same as giving up, he said. The point is to avoid being the forecaster who insists it will never rain, right before a thunderstorm hits. That doesn't mean every race is a toss-up: The model puts Wyoming at 99% Republican.
"We have a real public role, especially in polarized times, where people look at these forecasts," Hare said. The goal, he said, was a more complete accounting of where the question marks are.
Jack Rametta co-authored the paper while a postdoctoral researcher at UC Davis. He is now a political scientist and research fellow at the Mercatus Center at George Mason University.
