In fact, this year’s DNC was the latest incumbent convention in American history! That absolutely has to be taken into account when examining the president’s standing in the polls, and it means we would be wise to discount his margin by a little bit.
Third, Obama and Romney have basically been trading leads in the Gallup poll since May. The only postwar incumbent who did not pull away early in the registered voter poll and still won was George W. Bush, whose victory also happens to have been the narrowest margin for an incumbent since 1916. (Truman trailed in polls of national adults through the summer and fall of 1948.)
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.