If McCain does win the nomination, it will be Dole '96 all over again. Hillary/Obama will walk all over him in the debates because, like Dole/Kemp, he'll adopt a "let's play nice" attitude that only he will adhere to.
Also, playing hypotheticals here, McCain will likely choose Huckabee as his VP, which will be another strike against him because then the left will be able to point to the ticket and say, "look, it's an old guy who's survived cancer and a fundamentalist Baptist preacher, do you really want them in the WH?"