The polls are closing in South Carolina in about twenty minutes, so we’ll begin to see just how large a margin of victory Obama has racked up.  There has been talk of a possible Edwards surge into second, which I suppose might keep him slightly viable for another week.  More important will be the effect of an Edwards second-place finish on Clinton, who has clearly written off S.C., but probably wouldn’t be expecting to lose to Edwards.  Failing that, the margin of victory will be important for Obama, since there is already an expectation that he is going to carry an overwhelming percentage of the black vote, so he will need a gaudy Romney-in-Nevada-like result to put to rest the concern that he is becoming the 2008 Jesse Jackson in terms of his base of support.  Like Romney with the large Mormon turnout, Obama needs to be able to show that he would have been competitive had he carried a much smaller percentage of the black vote.  If not, he becomes the Democratic equivalent of Huckabee in one sense only: he starts to be seen as the candidate of one constituency.  Just as Huckabee’s Iowa victory has been discounted because of the large number of evangelicals who participated, Obama’s South Carolina victory, as many other observers have already said this week, will win him a good number of delegates but give him only a very small boost.