Our new Senate model has inched towards Democrats in September

On September 23rd The Economist published our first-ever statistical forecast of the battle for control of America’s Senate. It makes the Democrats modest favourites, giving them a two-in-three shot at re-taking the upper chamber. Had we launched our model two weeks earlier, it would have shown an even closer race, with a 60% chance of Democratic control. The biggest driver of this movement has been a change in its analysis of Maine, one of the most likely states to give Democrats their 50th vote in the 100-seat chamber.

With the benefit of hindsight, it looks as if our forecast was too kind to Susan Collins, the Republican incumbent, from the start. In states with little polling, our model relies on “fundamental” factors like past voting records, which favoured Ms Collins. She won re-election in a landslide in 2014. Donald Trump came within three percentage points of winning her state. Voters have historically rewarded centrists, and Ms Collins is the Senate’s most moderate Republican. And incumbency tends to be most valuable in small states with lots of secular white voters, such as Maine.