Jen Rubin captures the most important dynamic leading into South Carolina:
Thompson gave himself an opening in South Carolina and gave conservatives a place to jump from the Romney leaking ship. He may have scuffed up Huckabee sufficiently to allow either himself or McCain to win SC. If the latter he ironically would have done his old friend the greatest of favors. (Perhaps one he might remember when it comes to filling VP slots should he get that far.)
Fred Thompson could be in a place to split the South Carolina conservative vote, especially with this nasty anti-Mike Huckabee ad running.
A John McCain win in Michigan, followed by Thompson knee-capping Huckabee in South Carolina could lead to a pretty positive place for McCain.