Romney needs to figure out a new case for himself. And unrelenting attacks on the latest interloper to challenge his "inevitability" may become less effective because Romney has gone there so often.
Is it possible we'll see another round of the Mittens reinventing himself?
The worst part for Romney is that the best case against Santorum on the matter of electability involves the former Pennsylvania senator's positions on social issues, which are well to the right of the center. Yet if he goes after Santorum as too socially conservative, Romney risks casting himself to Santorum's left on social and moral questions. And it is precisely Romney's image as an inconstant conservative that created an opening for Santorum in the first place.
Romney has been for and against the same things so many times, every political portfolio known to mankind has been exhausted.
To slice the throat of his competitor, he's got to undermine who he now claims himself to be.
The game is at check mate.