Rick Santorum is aware that now he has closed on Mitt Romney in the polls, there will be more scrutiny and likely negative ads from the Romney camp. So he's doing something smart -- he's putting out a humorous ad entitled "Rombo" in which a machine-gun wielding Romney lookalike is shooting mud balls at Santorum (and missing). What this does is make Romney's negative ads the issue, and perhaps inoculate Santorum against them when they come (and they will). And, because it's semi-funny, he won't get perceived as bitter and angry the way Gingrich was when he responded to Romney's attack ads:
I remember a campaign twenty plus years ago in Wisconsin where a little-known candidate used humor against his better-known opponents in a race where the major candidates were slinging mud at each other. The little-known candidate with the funny ads ended up winning. His name was Russ Feingold.
The more Santorum can come off as a sunny Republican who doesn't get angry, the better off he will be. I'm not sure he can pull it off -- in the beginning of the race he often came off as pretty much of a sourpuss -- but maybe he's learned his lesson.