Why Clinton Attacks Obama So

Matthew Yglesias quotes one of his commenters making what I think is a fundamental point about Clinton's candidacy:

It's really strange... each time Hillary really goes for the jugular (fairly or unfairly), I am repulsed. And then 30 seconds later, I realize that that's the whole rationale for her candidacy! She (and Bill) will simply do whatever it takes to win.

By attacking Obama with distortions, she's trying to demonstrate that she knows how to play dirty, because this skill is supposed to be an asset in a general election. But she's counting on voters to value this ability as a positive while not counting it against her that she (and Bill) are playing dirty.