The Democratic Party was going to paint the Republican nominee as Bush III, regardless of who won the nomination. Thus, the Republican base did the smart thing and nominated McCain, who has a (undeserved) reputation for being an independent thinker.
Ever since we've known that McCain was going to be the nominee, the Democrats haven't been able to explain just how it is that John McCain is like Bush. I know there is substance to the argument because I've been paying attention to how little by little, McCain has sold his soul in return for the Republican nomination. But the Democrats haven't done a convincing job of equating John McCain to the failures of the Bush admininstration.
When we make the argument without speaking extensively of substantive issues suh as foreign policy debacles, or the general hands-off approach conservatives employ with respect to domestic problems, the Democratic narrative of painting McCain as another Bush is thoroughly unconvincing. Are we supposed to believe that McCain is responsible for our plight because he's old, and thus, by virtue, been in Washington longer than Obama? Or, are we supposed to believe that he's running for Bush' third term simply because they are of the same political party?
Academics know why it's intellectually appropriate to label John McCain as the bearer of the status quo, but middle America doesn't listen to these folks. The party has to make the Bush-McCain connection for us. After all, merely repeating "McCain is running for Bush's third term" doesn't make it so.