If he does, Nate Cohn expects it to be a small one:
Obama isn't unlikely to return to his post-DNC standing of 49 or 49.5 percent of the popular vote, which probably reflected an unsustainably poor image of Romney and post-DNC momentum (unless Obama's losses were almost entirely due to Democratic enthusiasm or response rates). With Obama likely to fall in a narrow band between his post-debate 47 percent and his pre-debate 49 percent, any gains would be slight and potentially difficult to distinguish from static. Of course, if Obama could get his number back near 49 percent, that would still be significant and potentially difficult for Romney to overcome.
Silver's guess about post-debate polling:
Throughout this election cycle, you would have done very well by predicting that the polls would eventually settle in at an overall lead for Mr. Obama of about two percentage points. Whenever his lead has been larger than that, it has come back to earth. But Mr. Obama has also rebounded at moments when the polls seemed to suggest an even closer race.