A successful last-minute Republican candidate. Or, of course, that perennial media fantasy, the brokered convention.
The big problem with Silver’s analysis is that it just uses polling data to tell us what we already know: Romney isn’t very popular among the Republican base, and there was a huge opportunity for an orthodox conservative to surpass him. But this doesn’t change the fact that this theoretical alternatives didn’t want to run, and in the extraordinarily unlikely event that one changed his mind now filing deadlines and delegate math preclude a successful run anyway. And once a frontrunner emerges, especially with a Democratic incumbent that candidate will become acceptable to most of the Republican base.