Who’s ahead in the mid-term race

By THE DATA TEAM

Read our leader about the 2018 mid-term results here, and see interactive maps and graphics of the election here.

AFTER two years of Republican government, Americans will go to the polls on November 6th to elect a new Congress. In addition to 35 seats in the Senate, all 435 seats in the lower chamber—the House of Representatives—are up for election. On current data our statistical model of the election gives the Democrats about a nine -in- ten chance of gaining a majority in the House . On average, we expect the Democrats to win a 25 -seat majority.

The model simulates the election in all 435 seats, 10,000 times. Some of those simulations result in landslide victories, others in narrow ones. The results of these simulations, and the size of majorities each party might hope for, are shown above. Small majorities can lead to gridlocked Congresses, with party leaders struggling to rally their members for every vote. Comfortable margins give the majority party scope to be bolder.

Although every seat is up for election, the real battle for control of the House is fought in a much smaller array of seats. We rate 276 seats, about two - thirds of them, as “safe”—where one party has a better than 99% chance to win. Another 76 are rated as “solid” (with a 90-99% chance to win). That leaves the remaining 83 seats to determine which party will control the House.

Democrats face a structural disadvantage in the race for the House. Democratic majorities tend to be larger than Republican ones. Because Republicans win by narrower margins their votes go further. This is partly due to “gerrymandering”: drawing electoral districts so as to favour a particular party. But much of the disparity is because the Democrats’ base of support—urban and non-white voters—tend to live near other urban and non-white voters.

However, the trend contains a hidden advantage for Democrats. Their large core of concentrated urban districts gives the Republicans few opportunities for gains. In contrast Republicans rely on districts where they have more competition for votes. A large shift towards the Democrats could see them make huge inroads into Republican territory.

For the Democrats winning the popular vote is not enough. Their structural disadvantage means they must receive many more votes than the Republicans to win a majority. In 2014, for example, the Democrats won a House seat for every 189,000 votes they received. The Republicans, by contrast, won a seat for every 162,000. In order to be favoured to win the House our model thinks the Democrats must win the two-party national popular vote by about 5.5 points. If as many people vote in 2018 as did in 2014, that would mean the Democrats need to win 4.1 million more votes than the Republicans.

The best measure of how the election is playing out across the country, and of whether the Democrats might indeed find such a large shift, is the “generic ballot”. This basic survey of voting intentions has been used since the 1940s and has a very good record of predicting the final national popular vote.

You can learn more about how our model works in our methodological article.

This is a new version of our model, incorporating newly-available district-specific data. Read about the changes here.