Who is going to win the US midterm elections according to the polls?

Americans are voting this Tuesday and the outcome of their vote will reconfigure both chambers of Congress: the House of Representatives and the Senate. While Joe Biden will remain president, the essential question is which party will have control of the chambers? If the Democrats lose one or both institutions, it will be difficult for the Biden administration to carry on with its legislative agenda in the two remaining years of the term. Here are the latest projections for the House and Senate.

Republicans are favorites in the House

Polling data indicates that the Republican Party could regain control of the House of Representatives, which has been in Democratic hands since the 2018 midterm elections. The following table shows the predictions made by five sources:

• According to a model based on polls by FiveThirtyEight, Republicans have an 82% chance of winning the House. This is more or less the same probability calculated by Metaculus, the prediction community.

These numbers make the Republicans a clear favorite, but it is not a foregone conclusion. Democratic control is still about as likely as rolling a six on a dice roll.

Remember that the entire House of Representatives is up for grabs on Tuesday, as is the case every two years when each of its members is voted on, district by district. In addition, in a midterm vote, it is quite common for the party that won the White House two years earlier to lose some backing, and on this occasion that would be the Democrats.

The Senate is a closer race

Democrats have controlled the Senate since 2020, but only by the slightest of margins. In addition, this chamber is not being completely renewed: only 33 of the 100 seats are at stake. The rest will remain the way voters decided two and four years ago (with some exceptions). Of the seats at stake, the Republicans are defending 21 and the Democrats only 12, which gives them some advantage.

Here is what the forecasts are saying:

• According to FiveThirtyEight and Metaculus, Republicans are slightly ahead in the Senate race, with an approximately 55% chance of winning the chamber, compared to 45% for the Democrats. This edge is closer to 67% according to the online prediction market PredictIt.

It is important to understand that these odds represent a very small advantage. All things considered, a 55% probability phenomenon fails to occur almost half the time. Distinguishing a perfectly balanced coin or one that returns 55% heads would require you to do a lot of tossing, because they are really alike. The Senate is that coin.

The trend favors Republicans

The above data reflects the situation two days before voting. But the trend over time is relevant. The following graph shows how the forecasts have changed in the last month according to the FiveThirtyEight model: the data shows a tendency favoring the Republicans.

• In early October, Democrats had a 30% chance of holding on to the House, and they were also the favorites to win the Senate, but in recent weeks their chances have dropped to nearly half of what they used to be.

In reality, all the forecasts have moved along the lines signaled by the polls, as shown below:

In short: Control of neither the House nor the Senate is decided yet, but Republicans have a good advantage in the first and a small one in the second.

The cross-summary provided by FiveThirtyEight is useful: 55% of the time the Republicans will gain control of both chambers, 14% of the time it will be the Democrats who will maintain both, and in another 31% of possible universes, there will be a distribution of power: the Democrats will keep the Senate and lose the House of Representatives.