When voting for any office, I always worry about whether or not my candidate is going to "grow in office." ("Grow in office" is a phrase that means "become more liberal." Becoming more conservative not only never happens to anyone in a position of power, but it is also never considered growth.) It seems as though it can happen to anyone.
But with John McCain, we already have the answer. He ran against George W. Bush for the nomination in 2000, discovered that he could earn the press's love and attention, and suddenly became a "maverick." Far from being the only person who can unite the Reagan conservatives, John McCain declared war on them. He built up his independent image by hinting at (but never actually) switching parties. I don't want my presidential candidate to think about what the New York Times says about him, except to relish their hatred of him. John McCain ain't that guy.
There's a reason that nearly every column supporting McCain cites his 82 lifetime rating by the American Conservative Union. His lifetime rating is meaningless; what is his rating since 2000? I'm betting it isn't quite so good. From immigration to judges, from taxes to the environment, John McCain has had only kind words for liberal colleagues and only disdain and often outright malice toward conservatives.