As a ridiculously close election enters its final stages, the balance of power in the United States appears to be in complete jeopardy. In 538's forecast for the presidential election campaign, Vice President Kamala Harris and former President Donald Trump are neck and neck. The US House of Representatives is also on a knife edge. But the result in the US Senate is not nearly as exciting.

Today 538 reveals our forecast for the upper chamber. At launch, Republicans have an 87 in 100 chance of winning a majority (including cases where the Senate is split 50-50 but Trump wins the presidency, leaving Vice President JD Vance as the tie-breaking vote in the Senate) . The chances of Democrats maintaining their majority are 13 in 100: about the same as the chance of rolling a dice and getting a one.

The Democrats are faced with a bad map

Our forecast reflects the appeal of polarization in America today. To put it simply, Democrats face a terrible map. Although the party (plus the four independents who align with it) currently has 51 seats in the Senate, one is in West Virginia, which is virtually guaranteed to flip to Republicans with the retirement of Sen. Joe Manchin this fall . This drops the Democrats' starting point to 50. The party then has to defend two more seats in reliably red states.

First there is Ohio, where Democratic Senator Sherrod Brown is running a tough 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 Democrats. In presidential election years, we wouldn't expect one party to have such a large turnout advantage, nor should we expect many split-ticket votes.

In fact, a model based only on “fundamentals” such as Ohio's partisanship and Brown's strength as a candidate would predict that he will lose by 1.1 points, although with a lot of uncertainty in that estimate (the confidence interval is approximately R +15 to …). D+14).

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

If we combine the polls with the fundamentals, our overall forecast for Ohio is a mistake, with a 56 in 100 chance of Brown winning.

Then there's Montana, one of the last places in America where you'd expect to see a Democratic senator. But Senator Jon Tester is no ordinary Democrat. Tester, who was also elected in the 2006 Democratic wave, describes himself as a “dirt farmer” who worked as a teacher, butcher and state senator before running for federal office. He has the kind of rural charisma and local connections that helped him escape the state's partisan politics and win office, even as the rural, conservative state fell short of electing the Republican presidential nominee by 2 points a year slipped to 16 points in 2008 in 2020.

But the tester's luck may run out this year. It's not just the fundamentals that speak against him, but also the polls: Republican challenger Tim Sheehy leads Tester by around 5.4 points in the poll average of 538.

That's a fairly wide margin that withstands most polling errors and confirms the diagnosis of fundamentals and experienced racial evaluators. Therefore, our overall prediction gives Montana Sheehy an 89 in 100 chance of winning.

The rest of the field

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

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

Northern battlegrounds are more likely to surprise Republicans. In Michigan, Democratic Rep. Elissa Slotkin has a 77-100 chance of defeating former Republican Rep. Mike Rogers and succeeding outgoing Democratic Sen. Debbie Stabenow. In Wisconsin, Democratic Sen. Tammy Baldwin has a 78-100 chance of beating Republican investor Eric Hovde. And in Pennsylvania, Democratic Sen. Bob Casey Jr. has a very similar 79-100 chance of defeating Republican hedge fund manager Dave McCormick.

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

In contrast, Democrats don't have many opportunities to flip Republican seats, and when they do, they are underdogs. In Florida, Republican Sen. Rick Scott has an 83-in-100 chance of winning re-election against former Democratic Rep. Debbie Mucarsel-Powell, and in Texas, Republican Sen. Ted Cruz has an 82-in-100 chance to defeat Democratic representatives. Colin Allred. Still, in our estimation, that means Democrats have a slightly better chance of securing one of these seats than retaining Montana. Since Democrats will most likely need to win at least one of these seats to retain their Senate majority, it might be wise for Democrats to do everything they can to defeat either Scott or Cruz. Sometimes the best defense is a good offense!

Another problem is the regular Senate election in Nebraska.* Normally the Cornhusker State is solidly red, but independent Dan Osborn has made life difficult for Republican Senator Deb Fischer: The incumbent leads Osborn by just 0.9 points in our polling average to. However, our forecast still gives Fischer a 95-100 chance of winning re-election, thanks to very Republican-leaning fundamentals.

A hypothetical Osborn victory would complicate the Senate majority calculation. We don't know whether he would represent a Republican influence or a Democratic gain: he is running as the de facto Democratic candidate in this election, but he has said he will not run either party, and he has enough conservative views that he could even plausibly side with the Republicans, especially if they hang out with carrots and plum committees. Our forecast takes this uncertainty into account by randomly determining which party Osborn would face if he wins. A third of the time, we count him as a Democratic senator to predict chamber control; in a third of the cases we count him as a Republican; and in a third of cases we assume that he remains a pure independent and does not belong to any party.

This election will determine the long-term fate of Democrats in the Senate

Thanks to their very disadvantageous situation, it was clear from day one of the campaign that the Democrats were at a disadvantage in the 2024 Senate elections. But it's not just 2024: The way the Senate as a whole is structured gives Republicans a built-in advantage. That's because the Senate allocates the same number of seats to each state, and because rural areas vote more Republican, there are more red-leaning states than blue-leaning states. This bias has increased over time, particularly as split-ticket voting has become less common. According to a 538 analysis, the median Senate seat was 5 points lower than the country as a whole after the 2020 election — meaning Democrats will need to win every blue state as well as a handful of light red states to gain control of the chamber.

That means that no matter how bad the 2024 map may be, the 2026 election may not be much friendlier to Democrats. They will defend four seats in states where President Joe Biden had a lead of less than 8 points in the 2020 election (Georgia, Michigan, Minnesota and New Hampshire). By contrast, Republicans will defend three seats in states that Trump lost or won by fewer than eight seats (Maine, North Carolina and Texas).

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

That means that without a significant realignment of party coalitions or a revival of split voting and denationalized races, Democrats' current majority in the Senate could be their last for a while. And Republicans, on the other hand, are looking forward to a bright decade of opportunity in the Senate across the country.

Footnotes

*Nebraska is also holding a special election for its other Senate seat at the same time after former Republican Sen. Ben Sasse resigned last year. It's not competitive.