Electoral College Forecast--September Polls

Back in April I wrote a post about using September polls to forecast election outcomes in the states. The general finding from the work I've done with Jay DeSart is that candidates generally win states in which they hold a lead in September polls and always win when they lead outside the margin of error. Jay and I use this information, along with a lagged vote variable and a control for national trial-heat polls in a state-level forecasting model.

Jay has done the heavy lifting on this model and has posted the details of the 2008 forecast on his web page. The results are summarized in this map.


Based on the September Poll Model, we project an Obama Electoral College win with 336 votes to McCain's 202 votes. Projecting the national popular vote based on predicted state margins and assumed state-level contributions to the national vote, we get Obama with 51.8% of the two-party vote.

Coming attraction: We will soon be unveiling an October Model that tracks changes in projected electoral votes as new polls come out.

Update: Missouri but not Virginia? Several readers have asked in comments why we are calling Missouri but not Virginia for Obama. Good question. The short answer is because that's what the model projects. Sometimes when you run a model like this you get results that make you scratch your head and wonder what's going on. But if you look under the hood of the model, it's somewhat easier to understand.

As described above, the model considers two state-level variables, the average poll result from September and a lagged vote variable from previous elections (averaged over four prior elections). On the lagged vote variable, Democrats have done better in Missouri than in Virginia, so this would push it toward the Obama column. On the poll variable, Obama did better in Virginia than in Missouri but not by enough, given Virginia's voting history to call the state in his favor. While Obama was leading in Virginia polls by the end of the month, he trailed McCain earlier in the month, and the month-long average was just about dead even. In the end, the data from September polls and lagged votes together pushed Missouri just into the Obama column and Virginia just into the McCain column.

No doubt part of the surprise here is due to Obama's strong performance in recent polls in Virginia. But remember, his biggest gains have come in the first week and a half of October and are not incorporated in this model. As promised, an October model is coming soon to a blog near you!