One other quick thought about the South Carolina polling: Depending on which poll you consult, there were still a decent number of undecided voters heading into today.
It's hard to discern a pattern among undecideds in automated polls versus live-interview polls (see my previous item for why this would matter). But, when I peruse the recent polls, my murky, impressionistic conclusion is that there seems to be a slightly higher percentage of black undecideds than white undecideds. Is there anything we can say about how that might play out?
Probably not. But one extremely tentative thought is that there may be something analogous to an incumbent effect at work here, which would cut against Obama. That is, black voters in South Carolina have presumably given him a long, hard look by this point. If they're still genuinely undecided headed into Election Day, they probably have some unresolved concerns, which means they're probably not going to vote for him. So you might expect black undecideds to break heavily for Hillary.
Of course, the huge caveat here is that we really have no idea how many black undecideds are out there. As I said, the number fluctuates pretty wildly according to which poll you read, and there's always the possibility (again see my previous item) that the voters who say they're undecided are lying to pollsters.