Unlike Perry, Santorum is articulate. Unlike Cain, he has political experience and knowledge of public policy. Unlike Gingrich, he has a personal life that seems to be above reproach. Romney has no advantage over Santorum in any of these respects.

Romney has cited his victory in a statewide election in a blue state to make the case that he can appeal beyond the party’s base. That’s something neither Perry nor Cain nor Gingrich could say. Santorum, on the other hand, won statewide in Pennsylvania (BEESPA), which hasn’t voted for a Republican presidential candidate in 24 years. And he did it twice.

Santorum’s strengths have left Romney resorting to weak attacks. Romney has pointed out that Santorum, like most Republicans who served in the Senate with him, supported such big-spending initiatives as the expansion of Medicare to cover prescription drugs. But Romney’s own health-care law in Massachusetts undercuts any attempt to portray other Republicans as soft on big government.

More recently Romney has gone after Santorum for having supported earmarks. But Santorum no longer supports them. If having previously taken positions that conservatives dislike disqualifies a candidate for the Republican nomination, Romney might as well quit the race now: His past liberal positions easily outnumber Santorum’s.