The main reason McCain has had so much success so far is because the "conservative" vote has been so fractured. In the beginning we had Hunter, Tancredo, Romney, Larry Flynt, Huckabee, and Aryan Paul vying for the Republican right. On the left, it was only Giuliani competing with McCain, and he wound up taking as many "conservatives" as he did liberals (because of his tough rhetoric on Iran and terrorism).
Now, the situation is even worse: on the right, we still have Huckabee and Ron Nazi Paul to suck votes from Romney, and an unopposed and universally worshipped McCain.