Noam is absolutely right that John Edwards's miserable four percent showing raises the question of whether he stays in the race through South Carolina. Because suddenly one of the most convincing rationales for a continued Edwards campaign--the slow accumulation of delegates that might make him a convention kingmaker--is probably now kaput. A third-place candidate can indeed rack up delegates in states where they are proportionally awarded. But my understanding is that so many places have a viability threshold of fifteen percent, a level Edwards isn't likely to meet after today's disaster, that the scenario pretty much off the table.
Going back to South Carolina, an interesting question is whether and how Hillary supporters might try to pressure Edwards out of the race. Given that his role fundamentally involves the racially-charged question of splitting the white vote (see Tom Edsall on its broader implications in the South), they'll have to tread carefully. Still, the temptation will be great, and we may be in for another round of uncomfortable identity politics.
Update: Edwards is pressing on. His campaign's statement:
“The race to the nomination is a marathon and not a sprint, and we’re committed to making sure the voices of all the voters in the remaining 47 states are heard. The nomination won’t be decided by win-loss records, but by delegates, and we’re ready to fight for every delegate. Saving the middle class is going to be an epic battle, and that’s a fight John Edwards is ready for.”