One thing to keep an eye on as we head toward South Carolina is Barack Obama's share of the white vote there. It's a key number for three reasons. First, Obama should do well enough among African Americans that if he can win close to, say, 30 percent of white voters, he'll win by a significant margin overall. Second, Obama wants his victory to be as broad-based as possible to show he'll be formidable in states without large African American populations.

Finally--and this is related to the previous point--expectations are forming in such a way that an Obama win in South Carolina, while certainly meaningful, would not come as a huge surprise. As a result, it won't give him the momentum boost he'd like heading into February 5. If, on the other hand, Obama can win nearly a third of the white vote, that will exceed expectations and could generate enough media buzz to provide an extra boost.

Obviously, one key variable here is how well John Edwards does. Recent polls (see here and here) show him taking 25-30 percent of the white vote (and a negligible portion of the black vote), compared with around 20 percent for Obama and around 40 for Hillary. If, as expected, Edwards's number drops following his lopsided defeat in Nevada, Obama could in principle pick up many more white votes. 

Now, the conventional wisdom--which I've echoed before--is that an Edwards flameout helps Hillary, since she'd take more of his white voters than Obama would. But, assuming Obama still wins, an Edwards flameout could actually help him in the post-game spin, by getting him over some impressive absolute threshold even if Hillary ends up winning more Edwards voters overall.

For example, suppose former Edwards supporters split 60-40 Hillary-Obama, and Edwards's support falls by half relative to the most recent polls. While the net effect would be to pad Hillary's vote-total, it would give Obama another 5-6 percent of the white vote, and could make his win look look more impressive.*

Anyway, just a thought.

*Obviously, Obama has to take a significant portion of former Edwards supporters for this to work out...

Update: Reading Paul Krugman's column today about the Obama-Reagan flap made me wonder: How many Reagan Democrats are there in South Carolina? Probably not an insignificant number. (Though a lot of the South Carolina Democrats who voted for Reagan probably became Republicans long ago...)

--Noam Scheiber