At their next debate, when Romney reverses himself on a position that he had previously espoused, Obama should say, "Wait, is that you, Mitt? Keeping track of your positions is just about impossible. I half expect you to peel of your face and have it be Tom Cruise under there. Or maybe a lizard, I don't know."
This does a few things:
1) It holds Romney to his nomination-stage extremism and exposes his pandering shiftiness.
2) It points out how unlikable Romney is on a physical level.
3) It calls attention to the Reptoid agenda that is at the heart of the GOP platform.