Now, I don't know how actively the Romney team is going after other candidates, but you know he's doing something right when he's been the frontrunner for a year but has never faced the intense negative criticism that the Anti-Romney of the month has.
He's never faced intense negative criticism because the media wants him to win the primary. They want him to win the primary because otherwise Obama will win the general election in a rout, which is obviously not very exciting (and thus does not sell as well).