I promised you a prediction challenge. Hang on a couple of days – too much discussion arising from the House prediction.
A number of concerns have been expressed about the House calculation, which requires more assumptions than the Senate or Presidential calculations. One, concerning the Monkey Cage model (and others like it) is resolved by pointing out that their model predicts a very broad range of outcomes between R+13% and D+9%, with which my more precise calculation is fully consistent. Another is that district-level polls may be more accurate than the generic ballot question. That will be true…once fresh district polls are available.
Still not fully resolved is the predictive accuracy of September generic Congressional preferences. On Election Eve, it is fairly accurate. But where will it go between now and then?
In my calculation, I assumed that it could move in either direction. Kevin Drum suggests that it might tend to move toward the Republicans. Three of his five examples are midterm elections. During a midterm Congressional election season, opinion moves against the sitting President’s party:
In midterm years, the amount of movement against the incumbent President is pretty significant post-Labor-Day. The above figure, from a paper by Bafumi, Erikson, and Wlezien does not address on-years like 2012, but it does show the kind of analysis that is possible.
There is a well-known coattail effect in which the President’s party does better than in off-years. But that does not answer the question of movement, which is the relevant concept now that we have a precise reading of current conditions. It would make sense for movement to be toward the Presidential winner’s party. Indeed, Kevin Drum says that there was movement of 3.3% towards Republicans in 2004 (Bush re-election) and 2.5% towards Democrats in 2008 (Obama election). If true, the probability of a Democratic takeover probability would be somewhat higher than the 74% I gave. Although we only have information for two elections at present, the question seems worth pursuing.
With apologies for leaving this issue not fully resolved, for the time being I stay with the assumption that future movement could go in either direction. This leaves the prediction unchanged: a likely (74% probability) Democratic takeover of the House in November.