Liberal discontent notwithstanding, conservative Democrats are the real weak link in
Conservative Democrats are ultimately a bigger threat to Obama's reelection prospects than liberal ones. They don't necessarily make a lot of noise about it when they're unhappy- they just go out and vote for Republicans. Liberals on the other hand really have nowhere to go- they can stay at home or vote for Ralph Nader but ultimately that's just going to get them someone who makes them a lot more unhappy than Obama. It's not a pleasant reality, but in our two party system that's just the way it goes- conservatives definitely have more leverage than liberals within the Democratic coalition and that's why they so often get their way despite their smaller numbers.
To go back to an earlier point, the most likely outcome of a credible primary challenge to Obama, even from the left, is a more conservative presidential nominee. Either Obama goes right to counter his challenger's influence on the left, or his challenger goes right to make up for Obama's strong support from African Americans.
As for conservative Democrats, there really isn't any way of getting around their electoral leverage; large Democratic majorities in the House and Senate are impossible without conservative Democrats, and ambitious liberal legislation can't happen without buy-in from those same members. Obama's habit of pre-concession is incredibly annoying, given the strong position of conservative Democrats, it's probably a precondition to keeping their support. Which is another way of saying that liberals are probably misfiring by focusing all of their anger on the president.
-- Jamelle Bouie