It's no exaggeration to say that the Romney campaign has developed a reputation for political ruthlessness. In Florida, with the help of super PACs and a massive fundraising advantage, they crushed Newt Gingrich-they drove him from the state and relished in the lamentations of his supporters.
With Rick Santorum, they plan to repeat the performance. The pro-Romney super PAC, Restore Our Future has already released its first ad against Santorum, which assails him for votes to increase the debt ceiling and spending. The ad includes a Romney surrogate, former Missouri senator Jim Talent, who attacks Santorum for his votes on legislation like the Medicare prescription drug benefit.
The problem, as you can probably see, is that these attacks are bloodless and unconvincing. It's hard to think of a senator who didn't vote to raise the debt ceiling during the Bush presidency; most Republican members of Congress were eager to sign off on Bush's priorities, even if they involved massive spending. Attacks on Santorum's record are unconvincing when they come from a man whose career has been defined by his devotion to political expediency. There's no reason to believe that Romney would have voted differently if he were in Santorum's position.
Mitt Romney wants to destroy Santorum, but unlike Newt Gingrich, the former Pennsylvania senator doesn't provide much material to work with. The usual avenues of attack are closed off. Romney doesn't have the credibility to hit the darling of social conservatives from the right, and attacks from the left only reinforce the view that he faked (or at least, exaggerated) his conversion to conservative orthodoxy. Of course, Santorum isn't ideologically pure-in 1994, like most Republicans, he was a supporter of the individual mandate-but his digressions are offset by a decade-long commitment to the right edge of movement conservatism.
By contrast, as Ross Douthat points out, Santorum can attack Romney from both the left and right as a champion of blue collar workers, and as the genuine article for Republicans tired of compromising their ideals for "electability."
There's one other complication for Romney as he moves to secure his (somewhat tenuous) position in the Republican primary with attacks on Santorum: He looks like a bully. Mitt Romney is flush with cash, endorsements, and the support of Republican insiders. Not only does Rick Santorum run behind in each, but he has little hope of catching up. A full-throated attack looks bad for Romney, and could drive up his already high negatives. To wit, GOP insiders have already asked the former Massachusetts governor to go a little easy on Santorum (a sign, perhaps, that they're hedging their bets).
As for Santorum, it's clear that he plans to capitalize on his underdog status. Here's his latest campaign ad:
At the moment, this race isn't as competitive as it looks; Mitt Romney is still the favorite for the nomination. But there's space for a genuine challenger, and if Santorum can survive the attacks from Romney and his supporters, then it might turn out that he is the long-prophesied "anti-Romney."