If the past is prologue, the polling averages will be off by a few points again this year in many races, but that error may be predictable, at least averaged over many races. And if we could predict the errors, we could do a better job predicting the final margin. Below I show two different predictions for all Senate races not listed as Safe by DKE (except Maine). Please note these predictions are experimental.
Previously, I showed that in close Senate and Governor's races, on average, a tie goes to the Democrat in deep blue states and the Republican in deep red states. We can use a regression based on this and presidential results to predict what the final margin for this year's races will be, in conjunction with current polling averages. In theory, the average error for these regression-based predictions should be less than the average error of the polling averages, but the regression based predictions should still have some substantial misses in some states.
Instead of a regression, we could simply use the past performance of polls in individual states instead. The major drawback to this technique would be lack of data for prior elections. But here's an example of how it works: in Nevada, the regression tells us polls underestimate the Democratic margin by about two points. However, the actual record shows underestimations of one, four, and nine points in elections with a margin of less than 20 points since 2004. This is also confirmed by errors of five and three points in the presidential elections. For Nevada then, we calculate a state-based estimate by adding five points to the Democratic margin. (Details below.) Again, the range is pretty decent, so the final prediction could easily miss by a few points. Hopefully, the average error of the state-based estimate will be less than the average error of the polling average alone.
So here's the current polling averages, and the two different predictions. Using the state-based estimates, for races with polling margins of five points or less, Democrats get a boost in six races, while Republicans are helped in only three.
