The forecast model is not making any sort of adjustment for the effect that naming a running mate could have on the polls. Neither, apparently, are the bettors at Intrade. But that may be a mistake.
What if we did include a vice-presidential bounce adjustment?
If so, it would probably be calculated as follows: since the average vice-presidential announcement has produced a four-point bounce in the polls, subtract four points from any poll conducted in between the naming of the running mate and the party convention. In order words, we’d subtract four points from Mr. Romney’s numbers in any poll conducted since he named Mr. Ryan on Saturday.
If I do that and run the model again, it has a rather pessimistic forecast for Mr. Romney — giving him just a 24 percent chance of winning the Electoral College, rather than 31.3 percent as in the official version.
The intuition behind this is simply that, under this theory, it’s a bad sign for Mr. Romney that Mr. Ryan has produced a below-average bounce so far. Among the polls that allow for a direct comparison, Mr. Romney has gained an average of about one percentage point since his selection of Mr. Ryan.