Nate Silver created the above chart out of Rasmussen data and it shows a fascinating difference between Obama's relationship to his base and McCain's relationship to his. A pretty hefty chunk of Democrats hate Obama and are certain to defect to McCain. But an absolutely giant number of Republicans feel kind of "meh" about McCain. As Nate observes, this is a perfectly fine situation for McCain to be in if it's a close election, "But if the election doesn't look like it's going to be close, there could be a snowball effect in which Republican turnout is quite low."
I'd also say that this represents Bob Barr's opening to have an impact on the race -- all these Republicans with mixed feelings about McCain could, in principle, be drawn to a different take on conservatism.