The most effective argument that Romney has against Santorum is that Santorum is a one issue nutcase who will scare independents away in a fall election and squander the opportunity to "make Barack Obama a one term president" to use Bachmann's mantra.

Unfortunately since Romney is currently pretending to be just as nutty he can't really use that line of reasoning with primary voters.

"When the snows fall and the white winds blow, the lone wolf dies but the pack survives."
Winter is Coming

Now this is the Law of the Jungle—as old and as true as the sky; And the wolf that shall keep it may prosper, but the wolf that shall break it must die.
As the creeper that girdles the tree-trunk, the Law runneth forward and back; For the strength of the Pack is the Wolf, and the strength of the Wolf is the Pack.