I think the answer here is to attack his strength. Muddy the waters and contaminate his message.Muddy things up and contaminate stuff. Doesn't that just scream rat filth to you?
This course of action is necessitated by the fact that Mitt is just so gosh darn unlikeable. A recent Rasmussen poll found that he has the absolute worst positive-negative spread of any candidate running on either side. Add to that the fact that he's up against a war hero who spent years in a POW camp in Vietnam, being tortured for our freedom while Mitt was prancing about hectoring Europeans about the Nephites and Lamanites on a draft deferment and you're pretty much reduced to something like Operation: Rat Filth.
Go Mitt! I can't wait to see Operation: Rat Filth unfold!