McCain has vocally supported some of Bush's most controversial positions. It's not just that he's a Republican, it's that he actually wants to keep doing the boneheaded things Bush has been doing.
Look how happy he is here! It's cute!
EDIT:
Hm... I posted that picture above, and then started to notice how many "John McCain Nuzzling George Bush" photos are in Google Images.
"Babe... I got you, babe! I got you, Babe!"
Look out George!
I'm sorry, I think this one is somebody else...
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I'm being silly here, but there is a point. My point is, it's going to be HARD for McCain to distance himself from Bush, and his policies are only going to make it harder. The perception that he's Bush III is really not that far off base. He USED to be a maverick, but he slid right to capture an evaporating base, and it's going to hurt him in the end.