Would a primary challenge from the left serve any purpose for Democrats? I believe that no matter how far left on the political spectrum you are or how frustrated you might be with progress toward your ideals or what's perceived as campaign promises unkept, that challenging Obama from the left would be futile and risk doing a lot more harm than good. It's possible that a challenge forces Obama slightly further to the left, so when and if he were reelected, he would have campaigned on a platform further to the left, but it almost certainly wouldn't effect how he'd govern anyway. He won't be able to get as far to the left as he wants anyway, as the next Congress and subsequent ones will be further right than he is. So basically I'm saying regardless of how frustrated anyone on the left is, a primary challenge is a silly option. In a 2nd term he would move the agenda as far left as a center-right Congress would allow anyway. A primary challenge could only cause divisiveness among ranks, push Independents away, and possibly prevent Obama from beating the Republican challenger. It might be the only way the GOP could win. There's certainly a miniscule chance that a Dem challenger would beat Obama in a primary, substantially less than 1%. If they were able to beat him (say 1 in 1000), it would only be because the Dems were weak at that point, and that challenger would have no chance of beating the Republican.