He could, although I think it is unlikely. The only Kerry states I see possibly switching are New Hampshire and Wisconsin, and I don't think they will. If McCain does win, it will be with no more than 275 EV.
First, if Clinton does as well Kerry, she loses.
Second, PA looks like a McCain state. Clinton doesn't have to do that much worse than Kerry to loose it.
Third, a lot of the general has a chance of being impacted by the current racially charged campaign. Unless that is ended, very quickly, Clinton will not be able to get the same African American vote that past Democratic nominees have. I saw the same thing happen with Dukakis. (See #2.)
Now, to both Clinton's and Obama's credit, they are working on number three, and trying to defuse the situation. That has the real potential of weakening any chance either nominee would have. (I think it might have to be Clinton/Obama 2008, or vice versa, to solve that problem, but
that would be a winning ticket.)