McCain's cooked. He missed his last best chance to overtake Obama.
Had he voted against the bailout, he could have come out on the same side of the election's biggest issue as the vast majority of Americans, AND put Obama in the uncomfortable position of voting for a drastically unpopular Bush proposal. A "no' vote could have instantly demolished Obama's "Bush clone" attacks, made McCain's "maverick" appeal more believable, shored up his base, and appealed to many swing voters.
Instead, he fell in line with Bush AND Obama. In doing so, he gained no points with the undecideds. His base, which was temporarily enamored/deluded with the Palin selection, felt betrayed. And no one takes his earmark veto threats seriously now that he voted for this disaster.
McCain might have pulled it off had he voted against the bailout. Then he could hit Obama with a lot of "I told you so" now that it's not working. Personally, I think he was afraid that if he voted against it, the Republicans would have fallen in line with him, and it would have failed. Then Obama could do the "I told you so" thing against him, when the economy didn't rebound.
By voting for the bailout, McCain screwed up three different ways. First, because you can't play it safe when you're behind. Second, because you don't want to vote for something supported by only 7% of the population. But most importantly, because voting "no" would have been the right thing to do.