Debates are unique, unlike the highly choreographed campaign event

Incumbents usually are at a disadvantage, defending a record against a challenger critiquing four years of work. Obama will be trying to avoid the fate of Presidents Ronald Reagan and George W. Bush, who turned in flat debate performances in their first encounters with rivals. In the end, though, it didn't hurt either one as they both won re-election.

"Debating is a muscle that doesn't get used very often," said Alan Schroeder, a journalism professor at Northeastern University and the author of "Presidential Debates: Forty Yearsof High-Risk TV." ''Mitt Romney is better toned because he came off 20-plus primary debates. President Obama has not been on a debate stage in four years."

Debates are unique, unlike the highly choreographed campaign event or stump speech marked by over-the-top rhetoric. Schroeder said debates require a different dynamic — candidates need to be respectful, differing in opinion but avoiding any impression that it's personal.