To many political observers (say, the Weekly Standard‘s Jay Cost, who writes about this with foot-stamping regularity), all you really need to know to predict the outcome of the presidential contest is the incumbent’s job approval rating. If it’s “underwater” just before the election, he’s toast, and would be even if his opponent was Adolph Hitler (yes, this is an exaggeration, but only a slight one, of how throughly some people dismiss the identity of the challenger or the possibility of a “choice” election).

But as Nate Silver notes today, one of the unusual things about Barack Obama is that there is a consistent gap between his job approval standing and his “favorability” ratings, which typically means there are people who like him but don’t particularly like the job he’s done (it could also mean there are people who don’t think he’s done a good job but don’t think it’s his fault). In any event, we don’t really know that much about what the gap might signify:

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. There are a couple of reasons for this. First, the two sets of ratings are normally extremely close to each other. Unless you need extreme precision for something, your choice just won’t matter that much. And second, the historical record of approval ratings is a little richer. Pollsters take them somewhat more often than favorability ratings, using somewhat more consistent question wording, and these ratings are archived more completely than the favorability ratings are. So approval ratings tend to be used as a default in studies of the presidency.

Nate identifies two predecessors who also had favorability/approval gaps, Bill Clinton in 1996 (higher job approval than personal favorability) and Jimmy Carter in 1980 (the reverse). In both cases, their actual vote performance fell between the two numbers.

That could mean, says Nate tentatively, that Obama’s ultimate vote performance will be slightly higher than his pre-election job approval ratings, which is actually exactly what we are seeing in current polls. If it turns out that way, Jay Cost may have to be committed to a sanitarium, and we’ll know a lot more going forward about the relative value of these common measurements of a president’s public opinion standing.

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Ed Kilgore is a political columnist for New York and managing editor at the Democratic Strategist website. He was a contributing writer at the Washington Monthly from January 2012 until November 2015, and was the principal contributor to the Political Animal blog.