Can either candidate move the needle? It will be tough: A review of data since 1960 suggests that past debates have almost never directly and measurably changed the candidates' relative standings. That's admittedly a high standard, though, and at least some debates may have had more subtle impacts.

Debates have been held in 11 presidential races since 1960. We find just one after which the lead in presidential preference changed hands by a significant margin: In 1980, when Ronald Reagan uttered his "are you better off?" line. He gained 7 points in a post-debate poll.

Still, there are other cases in which debates (or post-debate evaluations) may have influenced campaign dynamics. Debates are, after all, an essential window on the candidates’ styles and grasp of the issues. They can seem to add or reverse momentum. And polls are only so precise.