Is Rick Santorum invulnerable to Mitt Romney's negative attacks?

After Newt Gingrich crushed Mitt Romney in South Carolina, Romney responded with a massive barrage of attack ads that appear to have all but ended the Gingrich campaign. Now Rick Santorum is the candidate threatening to block Romney's path to the nomination, inching ahead of him not only in new national polls from PPP and Pew, but also taking a big lead over Romney in his birth state, Michigan. Can Romney and his allied super PAC repeat their Newt offensive, burying Santorum under attack ads in the two weeks before Michiganders vote? Or is the socially conservative family man impervious to the Romney attack machine?

Santorum is covered in conservative Teflon: Newt was an incredibly easy target compared to Santorum, says Roger McShane at The Economist. "Gingrich left himself open to attacks from the Right" because of his ties to embattled lenders Fannie Mae and Freddie Mac, and on character issues, thanks to his "checkered marital history." But conservatives love Santorum's consistent, far-right stances, and "the squeaky-clean" former senator is really only "vulnerable to attacks from the Left" on social issues like contraception, homosexuality, and the like. That makes it awfully hard for Romney to successfully attack him in a GOP primary.

"Romney's Santorum problem"

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Romney can still attack on the economy: Santorum actually has "quite a few vulnerabilities" that Romney could exploit, especially on fiscal conservatism, says Daniel Larison at The American Conservative. While in the Senate, Santorum backed big-government programs like No Child Left Behind and a new Medicare drug benefit. Ripping this "Bush-era fiscal irresponsibility hits Santorum where he is weakest with conservative Republicans," with very little risk of backlash for the business-savvy Romney.

"Santorum is vulnerable on fiscal and role of government issues"

But attack ads aren't a long-term strategy: "To be sure, the Romney campaign has to start 'defining' Santorum," says Amy Walter at ABC News. But Romney has to tread carefully, and he has to start making a convincing "positive case for himself" and his candidacy. Mitt's "'nuke Newt' strategy" destroyed Gingrich. But going sharply negative also really hurt Romney among both Republicans and the independents he needs this fall. His favorability ratings plummeted. If Romney keeps going negative, he does so at his own peril.

"Why Mitt Romney can’t go nuclear on Rick Santorum"