I launched my state-by-state senatorial and gubernatorial predictions in the first week of August. You can find them on PredictWise updating as the data updates: senatorial and gubernatorial. Most important for the senatorial elections, you can find the expected balance of power on a separate table. I hope to add the district-by-district house election by October 1.
The method is the exact same as I described in this paper, forthcoming in the International Journal of Forecasting; with one big exception as there are no state-by-state betting markets, yet. Thus, the data is simplified to just fundamental data and polling data. The polling data, pulled from Huffington Post’s Pollster and RealClearPolitics, is transformed into an expected vote share and probability of victory using the model outlined in the aforementioned paper. I average the aggregated polling snapshots and then correct for both the anti-incumbency bias (incumbents poll too low compared to the results) and the regression to the mean (leads narrow as Election Day approaches) with a set of linear coefficients determined by past elections. The fundamental data (e.g., previous votes in the district, incumbency, and economic indicators) is transformed into an expected vote share and probability of victory using the model outlined in this paper with Patrick Hummel (forthcoming in Electoral Studies). We tested dozens of reasonable variables and used historical data to form the most efficient relationships between the fundamental data and expected outcomes.
The reason that I have not written yet on this is threefold. First, I find the fixation on the horserace months before the elections distracting. Second, I am working on a lot of other cool things! Third, the senatorial forecasts are not that much different than anyone else.
I could have gotten out dozens of small articles on the state of the senatorial election over the last few months, but, to be frank, when I contemplate politics, I prefer to focus on matters like our impending war with ISIS (or Syria) (i.e., foreign policy), the ongoing persistent, but sluggish recovery (e.g., economic policy), and major policy debates over gay marriage and gun control (e.g., social policy). We have elections to change (or preserve) policy, so it seems foolish that we focus so much time on elections and so little time on policy. Also, the fate of the senatorial elections are largely out of our hands, unless you are big investor of money (or time), but it is always meaningful to learn more about the major policy issues of the day! That brings me point two, which is a totally exciting project that I will announce in a few weeks that focuses more on sentiment and predictions around issues.
I was pretty stoked when I compiled the tables and saw that they looked pretty similar to everyone else (good sign that everything was running well), but, then again, that does not make it too meaningful to you; my senatorial forecasts just confirm what everyone else is already saying. First, binary control over the senate is incredibly close. The Republicans need 51 seats and the Democrats need 50 seats. The most likely outcome, at 21% is the Republicans getting 51 seats; they have a 60% likelihood of getting 51 or more seats. Second, I have 10 competitive races (i.e., currently between 10% and 90%): AK, AR, CO, GA, IA, KY, LA, MI, NC, and NH. The New York Times has 9 competitive races (they have Kentucky at 7% for Grimes (D) and I have her at 12%; they have New Hampshire at 9% for Brown (R) and I have him at 17%; they have Kansas at 24% for Orman (I) and I have him at 8%, but that may change once the real polling starts there and the courts rule on the fate of the Democratic candidate). FiveThirtyEight has all 11 reasonable races in the mix. Third, everyone has the same few key races: NC, AK, LA, and IA, with KS a wildcard.
Here is New York Times and FiveThirtyEight compared with PredictWise:
Tomorrow I will provide an overview of the gubernatorial elections. And, over the next few weeks I will jump into the key senatorial and gubernatorial elections in more depth, and focus on shifts in the elections as they occur.