This is a good point.
Romney's Likeability (a positive motivator for voters) is higher than Obama's by about 2 points. But that indicates whether someone is motivated to vote FOR a candidate.
However a Dislikeablity (a negative motivator) would indicate someone who would be motivated to get out and vote for the competitor. Even more important, when someone votes against a candidate, they are actually moving the vote count by two.
Simply not voting shifts the balance by 1. Voting against a candidate shifts the balance by 2.
In a 47 to 48 race, not voting would tie the race. (47 to 47). Voting against would cause the competitor to win (48 to 47).
Many 2008 Obama voters are voting against Obama, each shifting the balance by 2.