First, Obama is weaker than previous incumbents who went on to victory. 

The second point to keep in mind is that, yes, we are late in the season, but so was the Democratic National Convention

Third, Obama and Romney have basically been trading leads in the Gallup poll since May.

The bottom line: Historically speaking, this president is in weaker shape than any postwar incumbent who went on to victory, with the possible exception of Harry Truman; he is enjoying a convention bounce later in the cycle than any incumbent in the postwar era; and if he manages to win, it will probably be via a true squeaker, with plenty of twists and turns to come.