Split the vote -- 1/3 Rudy, 1/3 McCain. 1/3 Thompson - and one of them will win by a whisker, most likely McCain would benefit and end up being the nominee and we would end up with Hillary.
Rudy might possibly be a VP, (but he probably will not agree to that) but he won't be necessary for a win.
Neither Rudy, or McCains following is that strong.