Republicans have a good shot at flipping the Senate

As a ridiculously close election enters its final stretch, the balance of power in the U.S. looks totally up for grabs. In 538’s forecast of the presidential race, Vice President Kamala Harris and former President Donald Trump are locked in a pure toss-up race. The U.S. House, too, turns on a knife’s edge. Yet the outcome in the U.S. Senate is not nearly as suspenseful.

Today, 538 is unveiling our forecast for the upper chamber. At launch, it gives Republicans an 87-in-100 chance of winning a majority (including cases where the Senate is split 50-50 but Trump wins the presidency, so Vice President JD Vance is the tiebreaking vote in the Senate). The chance that Democrats hold their majority is 13 in 100: about the same as the chance of rolling a die and getting a one.

Democrats face a bad map

Our forecast reflects the gravitational pull of polarization in America today. The Democrats, simply put, are facing a terrible map. Though the party (plus the four independents who caucus with it) currently holds 51 seats in the Senate, one is in West Virginia, which with the retirement of Sen. Joe Manchin is virtually guaranteed to flip to Republicans this fall. That brings Democrats down to 50 as a starting point. Then, the party has to defend two more seats in reliably red states.

First, there's Ohio, where Democratic Sen. Sherrod Brown is in a tight race against Republican businessman Bernie Moreno. Brown last won election to this seat in 2018, when he won by 7 percentage points. But 2018 was a great year for the Democrats. In presidential election years, we wouldn’t expect one party to have such a stark turnout advantage, and we also shouldn’t expect a lot of split-ticket voting.

Indeed, a model based only on "fundamentals" like the partisanship of Ohio and Brown's strength as a candidate would forecast him to lose by 1.1 points, albeit with a lot of uncertainty around that estimate (the confidence interval ranges roughly from R+15 to D+14).

However, the polls are slightly more reassuring for Brown. He currently leads Moreno in our polling average of the race by about 1.6 points, which is better for Brown than the fundamentals but still well within the range of historical polling errors in Senate races (see: Maine in 2020). Plus, the polls in Ohio have been moving against Brown, who had a 6-point lead in mid-July.

Combining the polls with the fundamentals, our overall forecast for Ohio is a toss-up, with a 56-in-100 chance Brown wins.

Then, there's Montana, which is one of the last places in America you’d expect to find a Democratic senator. But Sen. Jon Tester is no ordinary Democrat. Also elected in the 2006 Democratic wave, Tester calls himself a "dirt farmer" who worked as a teacher, butcher and state senator before running for federal office. He has the type of rural charisma and local connections that have helped him outrun the state's partisan lean and win office even as the rural, conservative state drifted from voting for the Republican presidential candidate by 2 points in 2008 to 16 points in 2020.

But Tester's luck may run out this year. Not only are the fundamentals against him, but so are the polls: Republican challenger Tim Sheehy leads Tester by about 5.4 points in 538's polling average.

That is a pretty wide margin that is robust to most polling errors and confirms the diagnosis from the fundamentals and expert race raters. As a result, our overall forecast for Montana gives Sheehy an 89-in-100 chance of winning.

The rest of the field

With their two red-state seats in critical danger, and West Virginia all but gone, our model expects the Democratic Party to lose about 2.3 seats on average before considering any other races. But Democrats have plenty of vulnerable seats beyond just those three. They are defending five seats in tight presidential battlegrounds: Michigan, Wisconsin, Pennsylvania, Arizona and Nevada. Luckily for them, our model predicts all five of these seats are likely to stay in Democratic hands.

The two Sun Belt states look most likely to remain in the Democratic column. In Nevada, Democratic Sen. Jacky Rosen has a 91-in-100 chance of defeating Republican veteran Sam Brown. And in Arizona, Democratic Rep. Ruben Gallego has an 88-in-100 chance of defeating Republican former TV news anchor Kari Lake to fill the seat being vacated by independent Sen. Kyrsten Sinema.

The chances of a Republican upset are higher in the northern battlegrounds. In Michigan, Democratic Rep. Elissa Slotkin has a 77-in-100 chance of defeating Republican former Rep. Mike Rogers and succeeding retiring Democratic Sen. Debbie Stabenow. In Wisconsin, Democratic Sen. Tammy Baldwin has a 78-in-100 chance of beating Republican investor Eric Hovde. And in Pennsylvania, Democratic Sen. Bob Casey Jr. has a very similar 79-in-100 chance of winning over Republican hedge fund manager Dave McCormick.

While Democrats still lead in each of these three seats, it would take only a normal-sized polling error for Republicans to win at least one of these contests. That gives Republicans another path to winning the Senate even if Democrats manage to hold onto their seats in Montana and Ohio.

By contrast, Democrats don’t have many opportunities to flip Republican-held seats, and where they do, they are underdogs. In Florida, Republican Sen. Rick Scott has an 83-in-100 chance of winning reelection against Democratic former Rep. Debbie Mucarsel-Powell, and in Texas, Republican Sen. Ted Cruz has an 82-in-100 chance of defeating Democratic Rep. Colin Allred. Still, that means Democrats, by our reckoning, have a marginally better shot at picking off one of these seats than they do of holding onto Montana. Since Democrats very probably need to win at least one of these seats in order to hold onto their Senate majority, Democrats might be wise to go all-in to defeat either Scott or Cruz. Sometimes, the best defense is a good offense!

One other wrinkle is the regularly scheduled Senate race in Nebraska.* Normally, the Cornhusker State is solidly red, but independent Dan Osborn has made life uncomfortable for Republican Sen. Deb Fischer: The incumbent leads Osborn by only 0.9 points in our polling average. However, our forecast still gives Fischer a 95-in-100 chance to win reelection thanks to very Republican-leaning fundamentals.

A hypothetical Osborn victory would complicate the Senate majority math. We don’t know whether he would represent a Republican hold or a Democratic gain: He's running as the de facto Democratic candidate in this election, but he's said that he won’t caucus with either party, and he has enough conservative views that he could even plausibly side with Republicans, especially if they dangle carrots like plum committee assignments. Our forecast handles this uncertainty by randomizing which party Osborn would caucus with if he wins. One-third of the time, we count him as a Democratic senator for purposes of predicting chamber control; one-third of the time, we count him as a Republican; and one-third of the time, we assume that he will stay a pure independent and caucus with neither party.

This election will decide Democrats’ long-term fate in the Senate

Thanks to their very unfavorable map, it was clear from day one of the campaign that Democrats were at a disadvantage in the 2024 Senate elections. But it's not just 2024: The way the Senate is structured overall gives Republicans a built-in advantage. That's because the Senate awards an equal number of seats to each state, and by virtue of rural areas voting more Republican, there are more red-leaning states than blue-leaning ones. This bias has been increasing over time, especially as split-ticket voting has gotten less common. According to a 538 analysis, the median Senate seat after the 2020 election was 5 points redder than the nation as a whole — meaning that Democrats need to win every blue state, plus a handful of light-red states, in order to win control of the chamber.

That means that, as bad as the 2024 map is for Democrats, the 2026 elections may not be much kinder to them. They will be defending four seats in states that President Joe Biden carried by fewer than 8 points in the 2020 election (Georgia, Michigan, Minnesota and New Hampshire). By contrast, Republicans will be defending three seats in states that Trump lost or carried by fewer than 8 (Maine, North Carolina and Texas).

Then, in 2028, Democrats will have to defend five seats that Biden carried by under 8 points: Arizona, Georgia, Nevada, New Hampshire and Pennsylvania. Republicans will be defending only three seats that Trump lost or carried by fewer than 8: Florida, North Carolina and Wisconsin.

This means, without a significant realignment of party coalitions or a revival of split-ticket voting and denationalized races, the Democrats’ current Senate majority could be their last one for a while. And Republicans, on the other hand, are looking forward to a bright decade of pickup opportunities in the Senate across the country.

Footnotes

*Nebraska is also holding a concurrent special election for its other Senate seat after former Republican Sen. Ben Sasse resigned last year. It is not competitive.