Admittedly, Mitt blew it. He didn't get it right, but most candidates make their points and show how different they are from their opponent early on, and then try not to open up any wounds in the last month.
Try it different next time and we'll see if it works any better.
To defeat the King, you have to kill him. In electoral terms, you have to really destroy the President, make him a joke, expose him and make him a wounded duck. Then, people will flock to the alternative. Until then, they are afraid of being on the losing side.