Romney needs to start setting up the groundwork for the fall and for a change maintain it after the primary is over. He should moneybomb Pennsylvania, but neither his ads nor those of the Restore Mitt's Future PAC should mention Rick at all. They should either be positive ads about Romney or negative ads about Obama.
Romney's SuperPAC spent about half a million in almost entirely positive ads in Louisiana. It didn't improve his numbers one bit and Santorum side aired mainly negative ads and they were very effective.
Right now if I was Romney I would use the threat of dropping a proverbal a-bomb of ads on Penn and doing everything I can to win there unless Santorum gets out of the race. The other stick would come from the big money backers and Republican establishment that if Santorum wants to run in 2016 or for any other major position he needs to end it now. My carrot of course would be Santorum gets a prime time speaking slot at the convention, any debts paid off, and he gets a graceful treatment by the party for the rest of the year.
Romney can't run to the middle until Santorum exits the race and endorses Romney or until Romney reaches the magic number of delegates. He will reach the magic number too late in the game so he has to force one of the major candidates out. Paul is going nowhere as he is running to support his cause. Newt really is irrelevent at this point he doesn't know it yet. Though its impossible for the RNC to really squeeze him as he knows this is likely his last election anyway.
That leaves Santorum who is still relatively young and I think wants a long political future in the GOP. Romney needs to make it known that he is no Ford and Santorum is no Reagan. That Romney with the help of the party boses will kneecap Santorum politically leaving him too toxic to have any hope of a successful run four years from now if he doesn't get out this week.