Chris Cillizza has a good post up pointing out that the only two times Hillary Clinton has been knocked back on her heels this campaign--over her ties to lobbyists and her support for the Kyl-Lieberman resolution on Iran--John Edwards was behind it. Chris then takes up the question of who stands to benefit from this:
[W]hile recent evidence suggests that it is Edwards not Obama who is best carrying the anti-Clinton message, it may not matter all that much when the actual votes are cast. Due to the underlying dynamics of the race -- the organizational and financial strengths of Clinton and Obama -- Edwards may wind up getting very little credit for the efficacy of his messaging. In fact, Obama may actually benefit from Edwards strongly carrying the anti-Clinton message as it ensures voters are aware of the potential problems with her candidacy while not blaming Obama for breaking his pledge to not engage in traditional negative campaign tactics.
I think this is basically right, though I'd quibble with the idea that there's just one "anti-Clinton" message, as Chris suggests. There are actually two threats to Hillary at this point: The first is from the left--she's too moderate, too hawkish, too close to big business, too establishment. The second is on electability grounds--she's too polarizing, she'll rally the GOP base, she'll hurt down-ballot candidates in the South and West. Edwards has generally been leveling the first critique--and quite effectively, as Chris points out. Obama has partly been leveling the first and partly the second, neither one very successfully.
The problem, as I've noted before, is that this is a little backwards: Obama has a lot of credibility when it comes to leveling the first critique (he opposed the Iraq war from the get-go, hasn't spent a lot of time in Washington) and not as much credibility on the second (as magnetic as he is, we still have no idea how an inexperienced, African-American senator will play in a national election). Conversely, Edwards has a lot of credibility on the second critique (he's a charming white guy with a thick North Carolina accent) and less so on the first (he voted for the war, ran as a moderate in 2004). Which is to say, if voters end up rejecting Hillary because they think she's too moderate,* they're probably going to vote for Obama instead. And if voters end up rejecting Hillary because they think she's unelectable, they're probably going with Edwards instead. I think that's just baked in the cake to some extent.
So while Edwards really is making a very compelling case against Hillary from the left, I tend to agree with Chris--he probably ends up helping Obama somewhat. Likewise, when Obama subtly derides Clinton as too polarizing, he's probably helping Edwards.
Having said all that, if I'm Edwards, and I'm fighting for my life in Iowa, then I probably hit Hillary with anything I think will stick and worry about the rest later. (And, for the record, the Edwards campaign has begun to highlight his electability advantages during the last week or two.)
*I'd originally written "liberal" rather than "moderate" here.