How Many Post-War Presidents Have Lost Reelection Without a Primary Challenge?

It's a question worth thinking about when assessing Barack Obama's chances in 2012, because although history isn't predictive of the future, it does provide some picture as to how things tend to play out.

So what's the answer? How many times in the post-war period has a President lost reelection without facing a serious primary challenge? The answer is zero.

Taking the question from the other angle, how many times has a President in the post-war era lost a bid for another term in the face of a primary challenge? Three: Gerald Ford (challenged by Ronald Reagan), Jimmy Carter (challenged by Ted Kennedy) and George H.W. Bush (challenged by Pat Buchanan). At least one more President, Lyndon Johnson, opted not to run for another term in the face of a primary challenge (in his case, Eugene McCarthy).

One can intuit the reasoning behind this trend: A party divided leads to a party losing power. There is, of course, the question of causation, whether it is the division within the party that causes the President's weakness, or rather the President's weakness that causes a splintering of the party coalition. (Indeed, it's probably a bit of both.)

But it's still worth keeping in mind about 22 months out from election day that no President since World War II has lost a bid for another term without already having waged battle against a serious opponent from within his own party.