ALTHOUGH they lack the intense personal drama of a presidential race, America’s mid-term elections in November will be hugely important. Every seat in the House of Representatives is up for grabs, along with 35 out of 100 Senate seats. A Democratic takeover of either chamber would unleash a flurry of investigations into President Donald Trump and wreck his hopes of passing more conservative laws on a partisan basis. If the Democrats take the House, Mr Trump might also be impeached.

This year’s mid-term campaign is extraordinary in another way. It is expected to be closely fought. Thanks to Americans’ tendency to separate into like-minded communities and to deliberate gerrymandering, most individual House races are one-sided. Historically, control of the lower chamber has been a foregone conclusion as well. In every contest from 1954 to 1992, the Democrats won at least 232 seats, well above the 218 needed for a majority. Since 2002 the winning party has always claimed at least 229 seats, with the results predictable (mostly in the Republicans’ favour) months before election day.

By contrast, this year’s contest appears to be poised on a knife-edge. Because district lines favour the Republicans, most estimates suggest the Democrats could wrest control of the House if they manage to win 53-54.5% of the total votes cast for the two major parties. For most of 2018, pollsters have reported that about 53.5% of people who express an opinion intend to vote Democratic. The Republicans appear likely to keep control of the Senate, but a Democratic wave could also put it into play.

Partisans from both sides see encouraging auguries. Republicans cite the economy, which is entering its ninth consecutive year of expansion. Democrats point to their recent victories in special elections. They won a Senate seat in deeply conservative Alabama, a House district in Pennsylvania that Mr Trump had carried by 19 percentage points, and came stunningly close to winning control of the lower chamber of Virginia’s legislature. Democrats also note that Mr Trump is disliked by historical standards. Despite a recent improvement, his approval rating is much lower than his disapproval rating. Over the past half-century, this has previously occurred four times at this stage in a mid-term cycle (1982, 2006, 2010 and 2014). Each time, the unloved president’s party proceeded to suffer devastating losses in the autumn.

Some pundits think the House race is too close to handicap. But The Economist has tried to do so. We have created a statistical model to forecast votes for the House, which has been trained on every election cycle since 1942 and 6,500 historical district races. At the moment, we give the Democrats a two-in-three chance of capturing it.

Our model is explained fully on our website. It begins by calculating the probability that each party will win a specific share of the overall popular vote, drawing on opinion polls, special and off-year elections, the unemployment rate and the partisan polarisation of the electorate. Then it delves into individual district races, weighing things such as voting history, incumbency, fundraising and candidates’ ideological views. It runs thousands of simulations and reports the percentage in which one party holds at least 218 seats.

Spying a blue wave

The model makes the Democrats a clear but narrow favourite. It assumes that some historical patterns will persist. In the past, undecided voters have moved towards the opposition party as mid-term elections draw near. As a result, the model expects the Democrats’ lead in polls that ask about people’s broad preferences to grow from 6.4 percentage points now to 8.8 in November. It places a heavy weight on performance in special elections, particularly those for federal congressional seats held within a year of the vote, in which Democratic challengers have fared well.

The model expects Democratic candidates to be competitive in many ostensibly red seats with retiring Republican incumbents. Of the 240 districts won by Republicans at the most recent elections, incumbents in fully 41 have already announced they will not run for re-election in November. That is more than twice as many departures as the Democrats have suffered. It is far easier to pick off an open seat than it is to oust a sitting member of Congress. Furthermore, in February the Pennsylvania Supreme Court redrew the state’s congressional districts. The old map greatly favoured Republicans; the new one is expected not to.