Julian Zelizer is a professor of history and public affairs at Princeton University. He is the author of "Jimmy Carter" (Times Books) and author of the forthcoming book "Governing America" (Princeton University Press).

As the South Carolina primary approaches, Mitt Romney has been struggling to respond to the blistering attacks from Newt Gingrich and a super PAC that supports his candidacy.

Gingrich opened up a line of attack on Romney's past work at Bain Capital, depicting Romney as a heartless capitalist, a "corporate raider," who made his money off other people's misfortunes. "Their greed," says the narrator of Romney and others like him, "was only matched by their willingness to do anything to make millions in profits ..."

Romney spent the final days of the New Hampshire primary trying to respond. He tried to turn the attack on its head, presenting himself as defender of the market economy and Gingrich as someone who was unexpectedly sympathetic to "European" ideas about government.

Although the response served its purpose, Gingrich has exposed Romney's greatest vulnerability in the campaign against President Barack Obama. Gingrich took Romney's greatest strength -- his background as in the world of business -- and turned it into a weakness....