One thing that strikes me about the latest round of Palin bashing is that her critics on the left appear to be acting against their own self-interest. I expect almost all of them would rather see Obama re-elected than defeated by a Republican. I expect most of them believe, as I do, that Palin would be one of the easiest of the potential Republican nominees for Obama to defeat. If so, one would expect them to go as easy as possible on her until it is clear who the Republican nominee is going to be, and only go back to making fun of her when it is clear that doing so won't reduce her chances of being nominated. That does not seem to be what they are doing.
One possible explanation is that her critics on the left think their attacks will make Republicans more willing to nominate her. An alternative I find more plausible is that what we are observing is the usual problem of producing a public good. The left winger who refrains from attacking Palin, given a halfway plausible opportunity to do so, is giving up a private benefit—the fun and kudos of mocking the big bad witch. He is producing a public good—the public in question being all of the people who want to see her nominated and defeated. Public goods, for familiar reasons, are under-produced.
The argument does not apply to critics on the other side, Republicans who want Obama defeated, think Palin is unlikely to beat him, and would therefor prefer a different nominee. Their attacks on Palin make perfectly good sense in terms of their political objectives.