This article has been reviewed according to Science X's editorial process and policies. Editors have highlighted the following attributes while ensuring the content's credibility: 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." 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.
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.
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