Chait's guess:

Scarborough is correct that elected Republicans, and especially Republican presidential candidates, have shied away from attacking Palin on the record. There's an obvious collective action problem at work. The party as a whole would stand to lose a great deal if she captures the nomination. She runs a good 8 points worse against Obama than against a generic Republican or Mitt Romney. On the other hand, she's popular among Republicans, and a candidate who attacks her would put his own candidacy in grave danger. So the heavy lifting is going to have to reside with Republicans who aren't running for office.

Bernstein half agrees:

[T]his is probably a problem that will take care of itself over the next year or so.  Nothing will change for most Republican pols: they don't want Palin as at the top of the ticket, but they also don't want to risk attacking her and then facing angry primary voters.  But for those running for president, the situation is a bit different.  Right now, it makes little sense for them to attack -- Palin may wind up dropping out of the race before Iowa, or she may just fail to rally support above the 20% or so level she's at now.  If, however, she looks like a formidable candidate for the nomination by next fall (give or take a few months), then attacking her makes more and more sense.

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