From “Why Obama May Be Stronger Than His Approval Ratings,” Nate Silver, FiveThirtyEight:
“Right now, the Real Clear Politics average shows 50.6 percent of Americans with a favorable view of Mr. Obama, versus 45.1 percent with an unfavorable one. That contrasts with his approval ratings, which now show 46.8 percent as approving his job performance and 48.7 percent as disapproving it.
“In other words, there is a small slice of the electorate, about 4 percent, that has a favorable view of Mr. Obama, but does not approve of his job performance. Given how close the election is, the way they behave in November could be decisive. If the election is a referendum on Mr. Obama based on his approval ratings, it’s going to be very close. He may be a slight underdog, especially since some of the approval ratings polls are of adults or registered voters, which are generally a point or two more favorable to the Democratic candidate than those of likely voters. However, if it’s a referendum based on Mr. Obama’s favorability ratings, his net-positive score (plus 5.5) makes him look like the favorite.
“Is there evidence on whether approval ratings or favorability ratings are a better indicator of a president’s re-election chances? Actually, there’s not very much of it. Favorability ratings have received much less academic study than approval ratings.
“If you take an average of Mr. Obama’s approval ratings and his favorability ratings right now, based on the Real Clear Politics numbers, you will get a positive rating (approve or favorable) from 48.7 percent of voters, and a negative one (disapprove or unfavorable) from 46.9 percent.
“The net rating — plus 1.8 percent — almost exactly matches his current standing against Mr. Romney. The Real Clear Politics average of head-to-head polls has Mr. Obama with a 2-point lead on Mr. Romney, while our ‘now-cast’ (which is based only on the polls and does not look at economic factors) has Mr Obama projected to a 2.1-point lead.” Emphasis added.
I think this is where the GOP is shooting itself in the foot by nominating Mitt. If they had a nominee with high favorability, that would make Obama’s favorability less important, since voters would find both candidates likeable, and make Obama’s lower job approval more decisive.