Ross argues that Santorum is Romney’s most formidable adversary yet:

At the same time, though, Santorum’s persona, his record and his platform all have a populist tinge that plays well in states like Michigan, Ohio and Pennsylvania, where swing voters tend to be socially conservative but economically middle-of-the-road. (Hence the Michigan poll that showed him leading among independents and Democrats who plan to vote in that state’s open primary.)

This means that Santorum can play the same anti-Bain, anti-rich-guy, blue-collar card that Gingrich tried to play in New Hampshire and South Carolina – but subtly, implicitly, in ways that don’t make him sound like he belongs in Occupy Wall Street instead of the Republican primary.

Maybe Santorum could do this, but he’s made it clear for some time that he isn’t willing to make those attacks. When most of Romney’s other rivals were hammering him on his Bain record, Santorum came to his defense. Santorum is one of the most vocal critics of Obama for his supposed “class warfare,” and he has stated repeatedly that he believes America is a classless society. He and Huckabee may have some things in common, but Santorum definitely lacks the chip-on-the-shoulder, working-class dislike of Romney that Huckabee displayed in 2007-08. This is related to Santorum’s ideological thinking, which Noah has just described. Even when it would suit Santorum’s political interests and matches up with some of his policy proposals, he cannot break from the script that treats any criticism of corporate behavior as a left-wing attack on the free market. The one Romney rival who can draw the clearest contrast with Romney on these issues will not attack Romney this way. He can’t even fully exploit Romney’s biggest liability in Michigan, which was his opposition to bailing out the auto industry, because Santorum agrees with Romney that the industry should not have been bailed out. Santorum’s populism is very subtle indeed.