Just read a terrific essay from Andrew Sullivan. I'm tempted to paste the entire thing here, but I'll instead pick out some of my favorite passages:
In the end, his final concern is not national security. No one who cares about national security would pick as vice-president someone who knows nothing about it as his replacement. No one who cares about this country's safety would gamble the security of the world on a total unknown because she polled well with the Christianist base.
McCain has demonstrated in the last two months that he does not have the character to be president of the United States.
Sullivan gets bonus points for employing the phrase "mindless Rovianism" and for using the word enormity properly. (Hint: It doesn't mean large size—that's enormousness.)
As usual, I encourage you to go read the whole thing yourself.