Forecasts for the US Midterm Elections 2018

By Stephen Fisher.

The most striking thing about the forecasts for today’s midterm elections in the United States is that they have been much less talked of in the media than in previous campaigns. This is partly because in 2016 most of the forecasters put very high probabilities (90%+) on Hilary Clinton winning the presidency. (See here for a post-mortem.)

This post reviews the main statistical model based forecasts for the US House and Senate, with some discussion of the methodology and comparison with other forecasts. Overall, and as usual, there is not much variation between the forecasters in their central forecasts. They all point to the Democrats taking control of the house and the Republican retaining control of the Senate. The striking exception is a Gallup poll suggesting 50% think the Republicans will retain control of the House and only 44% think the Democrats will win it.

Despite the forecasts differing from the expectations of the American people, the forecasts appear to have been widely accepted in the media. So much so that some journalists suggest it will be a vindication of Donald Trump if Republicans maintain control of the House. However, if that happens it will most likely be despite a clear lead for the Democrats in the popular vote. In which case, it would be the electoral system, not Trump, that thwarts the Democrats. Meanwhile, if there are net Republican gains in the Senate it will be primarily because the Democrats are defending a big haul from 2012.

House forecasts

As James Campbell has noted, in all but three midterm elections since 1900 the President’s party has lost seats. Since 1950 the average loss has been 24 seats. The Democrats need to make net gains of 23 or more to take control.

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