I see Paul's well-organized and passionate supporters being decisive in the Iowa caucuses. Romney should still be strong enough to hang on to NH (but Paul's momentum and ideological compatibility there might push him to a strong 2nd place showing). After that, it's hard to say what will happen in SC and Florida. Both Romney and Paul are pretty lousy fits for those states, so it depends on which of the other "conservative" candidates are still left standing at that point. If Gingrich is still alive, he may be able to win in SC and save his campaign and then push forward to another victory in Florida. Predicting beyond that is pretty futile at this point.