As I noted last night, there are a number of reasons why Obama's likability numbers may have stayed artificially high. Now, however (assuming the poll is accurate), one of three things has happened: Either the President has degraded his brand through his nasty, desperate attacks on Mitt Romney; or people who may not ever have viewed him all that favorably now feel more comfortable admitting it; or people who have always liked him have nonetheless decided to vote against him, and are trying to resolve a cognitive dissonance problem elicited by their decision to vote against someone they have always personally liked.
Whatever the cause, Obama's much-vaunted favorability was really the last metric keeping him afloat. If it's really no longer operative, he's got reason for concern.