Lost in McCain's attack is the subtle admittance that Bush has become the Republican's Carter. While Democrats have accepted the fact Carter was a mediocre-to-bad president, have rank-and-file Republicans had their own come to Jesus on Dubya? Are they ready to accept that he'll go down in history as their Carter? That's the risk in McCain's attack on Obama: It works on one level, but if he chooses not to defend Bush's presidency and allows it to be painted as a failed presidency, will the base be comfortable with that?
This strikes me as one more reason McCain will do fine as long as polls show him close to Obama, but will implode the second they show him faltering. In this case, a lot of loyal Republicans still revere Bush and don't like to hear McCain take implicit shots at him, but will swallow their uneasiness because they know McCain can help them keep the White House. If it starts to look like he might not be able to, though, all the barely-repressed animus is going to come back with a vengeance, and he'll lose in a landslide.
I doubt that this election will be close. Either Obama or McCain will probably win it in a landslide, depending on whether or not Obama can fulfill his existential mission of explaining to the American people who he really is.
I think McCain losing in a landslide is more likely, not only for the reasons I mentioned, but because the strong anti-Republican sentiment in the country and the energy of the Democratic base gives Obama a big boost. But Morris is right--Obama has to succeed at defining himself in the most basic way ("American," "mainstream," etc.) in order to have a chance. Fortunately, he's got the money and the opportunity to do it.
Update: Chris has more thoughts on the Carter analogy.