McCain's approach in this election is an interesting reversal of GWB; and every time people (especially Democratic Senate colleagues) cite his flip-flops, they feed right into McCain's best chance of winning. Here's how I think McCain's playing:
GWB had solid support from the right wing, so his main message was to woo centrist voters (Uniter, not a Divider; Reformer with Results), while he reassured the Right much more subtly.
McCains's most solid support is from the center, so his main messages have been to woo the Right (Country First, Secure the Borders). At the same time, he's been working very hard, but unofficially, to reassure centrists. His campaign has made huge feints like "considering" Lieberman and the "Hillary supporter" who "accidentally" got McCain's abortion position completely wrong.
Then comes Kerry's speech last night...
Candidate McCain now supports the wartime tax cuts that Senator McCain once denounced as immoral. Candidate McCain criticizes Senator McCain’s own climate change bill. Candidate McCain says he would now vote against the immigration bill that Senator McCain wrote.
See the problem? It matches up precisely with McCain's unofficial campaign to reassure centrists. People under the spell of the Maverick Myth hear Kerry and think "well, maybe we'll get Senator McCain, not Candidate McCain" if he's President.
Once again, the Obama campaign folks are the smartest people in the room. McCain = Bush has been so prevalent as meme even before the convention(about 66% and holding) that when speaker after speaker hammered away at it on the first two nights, pundits didn't even notice it. That's the message that undercuts McCain's centrist appeal; makes him a Sidekick, not a Maverick (best line of the convention so far); and resonates with Obama's message of Change.