HOUSE UPDATE: Bottom Falls Out of Democratic Party

Based on polling data released over the last nine months, Republicans are expected to gain between 39 and 62 seats in the U.S. House of Representatives (with a 90 percent level of credibility). According to the model, the most likely outcome is about a 51 seat gain by Republicans, up from the previous estimate of 48 seats. If the polling holds steady, Republicans seem to be poised to retake control of the U.S. House of Representatives with the largest electoral landslide since 1994. In fact, based on a Monte Carlo simulation, the Republicans have a 95 percent probability of retaking the House.

A statistical model that analyzes the results of 15 independent polling firms indicates that Republicans lead Democrats by a margin of 3.3 percent on the so-called generic ballot--a measure of national voter preference between the two major congressional caucuses. This figure is substantially greater than the August estimate of 2.0 percent. The September estimate implies that, if the election were held today, Republicans would achieve approximately a 25 seat majority over the Democrats--a modest, but decisive advantage.

Over the next two months, I will update this model and estimate Republican electoral gains. The charts below will provide the change in voter preferences over time, along with the probability distribution of the number of seats gained by Republicans on election day. See my April 30, 2010 post for an explanation of my methodology.