In the end, he does lay out the ominous consequences in store for the president if this bogus trend turns into something more concrete. McCarthy in 1968 and Buchanan in 1992 weren’t victorious, Bai explains, but their primary challenges were enough to damage sitting presidents seeking re-election.

Such protests candidates don’t have to win more than a state or two to have an impact; they merely have to show up and sow division. It probably isn’t coincidental that none of the last four American presidents to face primaries while seeking re-election—Johnson, Gerald R. Ford, Carter and George H. W. Bush—survived to serve another term.

In other words, should the president’s progressive critics warm to the idea, it might not take a particularly credible primary challenge to weaken Mr. Obama’s chances for re-election. It might only take a challenge designed to do exactly that.

In other words: should this trend actually turn out to be a trend, and if it the trend turns to action, then the president might have to worry.

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Joel Meares is a former CJR assistant editor.