I have often wondered about McCain.

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With his legendary temper and vindictiveness, I KNOW he hates Bush and has not forgotten the 2000 primary smears.
He may have a desire, unknown even to him, to utterly trash the party that put this administration in power.
If he loses he goes back to the cushy Senate seat.
If he wins he will do some serious payback within the party ranks.
Either way he has hurt them.