The Weekly Standard's Jay Cost does an analysis of Gallup polling data going back about 60 years and finds that incumbents with approval ratings as low as Obama's don't get reelected. You cannot argue with his data, only with his conclusions. He concludes:
"Historically speaking, this president is in weaker shape than any postwar incumbent who went on to victory, with the possible exception of Harry Truman."
It may turn out that Obama will win reelection with the lowest approval rating yet. Or it may turn out that the polls showing Obama ahead reflect the "Bradley effect," saying you'll vote for a black candidate when you plan to do no such thing. This is something some percentage of people do lest they be perceived by the pollster as "racist."