They both need someone other than McCain to win in South Carolina. Jay Cost explains why:
Following Pew, it does not seem that Romney is the second choice of a plurality of Huckabee voters or McCain voters. The situation in Florida might be different than what Pew finds on the national level, but I doubt it is significantly so. My sense is that if Floridians bolt Huckabee after he loses South Carolina - a plurality will go to McCain, not Romney. Similarly, if they bolt McCain - a plurality will go to Giuliani, not Romney. Generally, Pew and other pollsters have found Romney in third or fourth place when it comes to second choices. Pew also finds that 20 percent of Republicans will never vote for Romney, making him more 'unacceptable' than McCain or Giuliani. In light of this, I think that what Romney needs is a nominal Huckabee (or Thompson) victory in South Carolina. It would keep the field as open as possible. If the Florida electorate is split four or five ways, Romney might be able to pull out a victory based on his current coalition - thus giving him an opportunity to expand it in advance of Super Tuesday. Ironically, Giuliani is in the same position.
In other words: Tomorrow's GOP results in Nevada and South Carolina will probably resolve ... nothing.