With all due respect, if McCain is at the top of the ticket, he will not need Republicans to defeat Hillary. He will get around 50% of Rs, 80% if Independents, and 30% of D votes against her. So that even if staunch conservatives stay home, he will win.
You're dreaming.
First, McCain's not going to get the nomination. Secondly, the media love affair with McCain ends the second he becomes the favorite to win the nomination if he ever got in that position. Did you see how quickly they attacked him over his comments to Obama? They know McCain has an explosive temper and all it will take is to get him to blow up (the media excels at that) at the right time (like during a debate) and they will put a Democrat in the White House. The idea that independants and Dems will stay with him after that is a pipe dream.