My gut...The incumbant always has the advantage.
Not only must Kerry convince people that Bush has done a poor job, but he must also then convince people that Kerry is himself better for the job.
Note though that it isn't enough to just be "better" than Bush. He must first convice people to throw Bush out while at the same time convincing them that he's not going to upset the apple cart on critical issues like the war on terror, appointments to the courts, etc...
Its a very tough balance faced any time you are taking on an incumbant. People may want a change, but do they want Kerry's kind of change?
Its one reason I like the idea of term limits: Change is good thing(Every once in a while, the apple cart needs to be tipped over to get rid of the bad apples).
Scott Evil