Our midterm predictions point to major gains for the Republicans

SIX WEEKS AGO, the Democrats looked like they were on track to pull off a remarkable triumph. The president’s party almost always loses power in midterm elections. Yet in mid-September The Economist’s statistical forecasting model suggested that they had an 80% chance of retaining control of the Senate. The contest is now neck-and-neck, with the odds slightly in Republicans’ favour (see chart). Where have the Democrats lost ground?