Average these four scenarios together and the probabilities come out to almost exactly 50-50. A month or two ago, when Perry and Romney appeared about equally likely to be the Republican nominee, it would therefore have been proper to think of the election as a toss-up.

With Perry having slumped in the polls, however, and Romney the more likely nominee, the odds tilt slightly toward Obama joining the list of one-termers. It is early, and almost no matter what, the election will be a losable one for Republicans. But Obama’s position is tenuous enough that it might not be a winnable one for him.