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Our Final Forecast In The Senate, House And Gubernatorial Races

Happy Election Day! The last of the polls are in, and it’s finally pencils down for FiveThirtyEight’s 2018 midterm forecast. Here are our final forecasts:

Senate

Republicans have a 5 in 6 chance (82.0 percent) of holding on to the Senate in the Deluxe version of our forecast (which we’ll be using as the default version of the forecast for Election Day). In other words, Republicans are a strong, but not insurmountable, favorite in the upper chamber. Any scenario from a three-seat Republican gain to a two-seat Democratic gain falls within our 80-percent confidence interval.

In individual races, Democrats are slightly favored to flip seats in Nevada and Arizona, although we emphasize that those races could still go either way. (For example, Democrat Jacky Rosen’s chances in Nevada are only 63.9 percent.) But Republicans are likely to topple Democratic Sen. Heidi Heitkamp in North Dakota, while Missouri is a toss-up (both Democratic Sen. Claire McCaskill and Republican Josh Hawley have a 1 in 2 shot of winning). Democratic Sens. Joe Donnelly of Indiana and Bill Nelson of Florida are also in potential danger, although both seats lean Democratic. And because we know you’re wondering: Republican Sen. Ted Cruz is a 4 in 5 favorite to prevail over his Democratic challenger, Rep. Beto O’Rourke, in Texas.

House

Democrats have a slightly better chance of winning the House than Republicans do of holding the Senate. Democrats are a 6 in 7 favorite (85.8 percent) to win a majority of seats. Their average seat gain is 36, which would result in a House of Representatives with 231 Democrats and 204 Republicans. But the model considers a wide range of outcomes as still well within the realm of possibility: We think there’s an 80 percent chance that Democrats gain somewhere between 20 and 54 seats.

Roughly, your expectation should be that a Republican upset in the House is about as unlikely as a Democratic upset in the Senate. However, it’s much less likely that we will see an upset in both chambers at once, because polling errors tend to lean in the same direction, so if Democrats perform better than expected in the Senate, they will probably also perform better than expected in the House. (The same is true for Republicans.) Our final model run estimates there’s just a 0.2 percent chance that we will have both a Democratic Senate and a Republican House. There’s a 14.1 percent chance that both the House and Senate will remain Republican and a 17.9 percent chance that both chambers return to Democratic control. The most likely scenario, at 67.9 percent, is that control of Congress will be split.

Chances each party will control one or both chambers

According to the “Deluxe” version of FiveThirtyEight’s final 2018 forecast

Senate House Probability of scenario occurring
Democratic Democratic 17.9%
Democratic Republican 0.2
Republican Democratic 67.9
Republican Republican 14.1

Governor

Finally, our model foresees that Democratic governors will preside over an average of 24.2 states in 2019, or eight more than the party’s current 16. That would mean that roughly 197 million Americans, or 64.4 percent of the country’s population, would have a Democratic governor. Democratic gubernatorial candidates are favored to pick up governors’ offices in Michigan, New Mexico, Maine, Illinois and Florida (a closely watched race between Democratic Tallahassee Mayor Andrew Gillum and Republican former U.S. Rep. Ron DeSantis). They’re also slight favorites in competitive races like Iowa, Ohio, Wisconsin and Nevada. In the final version of our forecast, Kansas is the sole toss-up race. Republicans are leading in the competitive races of New Hampshire, Oklahoma, Alaska, South Dakota and, finally, Georgia, where Republican Secretary of State Brian Kemp has a 2 in 3 chance of defeating Democratic former state House Minority Leader Stacey Abrams (though perhaps not until December).

Nathaniel Rakich is FiveThirtyEight’s elections analyst.

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