I have no real disagreement with Jamelle Bouie's claim that a primary challenge to Barack Obama from the left is a foolish idea, and that liberals would be better off putting their efforts into more productive things.
I do think his "two (disastrous) scenarios" for what would happen should such a challenge emerge overlooks the most likely outcome: nothing. No one would pay much attention, and the challenger would wind up being on the same forgotten page of history as the two Members of the House who challenged Richard Nixon in 1972.
The idea here is that primary challenges to sitting presidents -- like third-party candidacies -- are effects, not causes, of presidential weakness. Now, it's certainly possible that Barack Obama will wind up in deep trouble in twelve months, and if that's the case then a significant primary challenge is certainly plausible. However, as of today, Obama's approval rating (per Gallup) sits at a respectable 47%. Moreover, thanks to overall decent numbers and strong party polarization, his approval rating among Democrats has been hovering right around 80% -- and it's a bit higher among liberal Democrats.
Of course, it's absolutely true that a lot of activists are upset with Obama right now...or at least upset, and blaming it on Obama. And, as I said, one never knows what will happen next. But there's a long way from where we are now to even a Pat Buchanan (in 1992) level challenge, let alone a Ted Kennedy (in 1980) challenge. Right now, given the solid popularity Obama enjoys among liberal Democrats, it's unlikely that he would even have to bother organizing a campaign against a challenge from the left -- although I suspect he would anyway just to get some solid campaign time in November swing states Iowa, New Hampshire, and Nevada.