I agree: the Democrats won't vote for McCain, but many Republicans incorrectly think that many Democrats will swing to McCain because of McCain's "hero" status.
(Denny Crane: "I Don't Want To Socialize With A Pinko Liberal Democrat Commie. Say What You Like About Republicans. We Stick To Our Convictions. Even When We Know We're Dead Wrong.")
I believe the Dems would successfully portray McCain as a dangerous loose cannon, same as Goldwater. I think it would be an easy general election victory for any of the main Democrat contenders.