The Guardian put this question to some 'top US commentators'. Well, they didn't ask me. Unabashed, I shall vouchsafe my answer - and without regard to the bet I have on the outcome.
Yes, it's over for McCain. The reason is the economic crisis. There is no way McCain can evade the evident fact that this matured during Bush's presidency. It can be said, obviously, that the roots of the crisis lie further back, which is true. But there's a statute of limitations in such matters. Whatever mistakes were made earlier, during Clinton's incumbency, enough voters will hold the Republicans responsible for not forseeing the crisis earlier, and acting in a timely way to head it off, to ensure an Obama victory. An added bonus (thrown in at normblog, but not by those top US commentators) is a thought about how this affects Gordon Brown and Labour. He and they weren't even toast before the crisis; they were the crumbs on the bread plate. But Brown has an opportunity of more than a year, as McCain before November 4 does not, to present himself as the man who began to clear up the mess (if he succeeds in doing that). So it isn't yet over for Brown.