Editor's note: 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).
Princeton, New Jersey (CNN) -- 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.
Romney, Democrats will argue, embodies the kind of ruthless mindset that they say defines Republican policy, an indifference to the unemployed and chronic poverty, a mindset that has fueled growing inequality and economic hardship.
Although much of the criticism on Romney has centered on accusations that he is a "flip-flopper," Gingrich's words are most likely to dominate the campaign in the fall, if Romney is the nominee.
The former Massachusetts governor will provide a foil to Obama, who in recent months has been developing a theme about being the champion of the middle class. This can be a potent theme in the current economic environment. According to a new survey by Pew Social and Demographic Trends, the number of Americans who perceive conflict between rich and poor people has risen from 47% in 2009 to 66% in 2012.
Turning a candidate's greatest strength into his or her weakness has traditionally been one of the most effective ways to win an election.
In 1988, Vice President George H.W. Bush unleashed a torrent of criticsm about Gov. Michael Dukakis' claims about a "Massachusetts Miracle." While Dukakis aimed to make the state a showcase of his successful policies, Bush pointed to dirty water, dangerous furloughed convicts and economic problems as evidence of a "Massachusetts Mirage." By the end of the campaign, the state weighed down Dukakis' candidacy and become something that he had to explain.
In 1996, President Bill Clinton eviscerated Kansas Sen. Robert Dole's campaign. In the middle of the 1990s, when Clinton was still vulnerable after Republicans had regained control of Congress and investigations were continually raising questions about Clinton's ethics, Dole ran an ineffective campaign. His greatest strength, compared with Clinton, was his deep experience in Washington, his service in World War II and the mature outlook that came with age. But Democrats turned this against him. They argued that Clinton represented the future; Dole represented the past.
"Building a Bridge to the Twenty First Century," Clinton and Gore boasted about what they offered. Democrats portrayed Dole as a tired and dated candidate who didn't offer the nation a vision for growth.
When George W. Bush took on Vice President Al Gore in 2000, Gore had a lot going for him. Republicans were on the defensive after their effort to impeach Clinton backfired. The president's approval ratings were high. The economy was booming. Gore also brought a lot to the table. He had a deep resume in Washington and a firm command of issues such as Social Security, technology and the environment.
Bush turned Gore's strength into a weakness. He and his campaign depicted the vice president as arrogant and condescending. When Gore spoke policy, Bush rolled his eyes and characterized Gore's economic numbers as "fuzzy math." The Republicans took his accomplishments, such as his role in promoting legislation such as the High Performance Computing and Communication Act of 1991, and used it as evidence that Gore was a serial exaggerator who could not be trusted. Gore's clumsy phrasing that suggested he literally invented the Internet provided the GOP with fodder for their attacks.
Bush did the same thing with Sen. John Kerry in 2004. During the Democratic Convention, Kerry gave a military salute to the delegates and spoke of his service during Vietnam as a way to distinguish himself from Bush, who served in the National Guard, as well as Vice President Dick Cheney, who received deferments that protected him from the draft. Bush and Cheney were claiming that Republicans were the party that was tougher on defense.
The Republicans went to work. They focused on Kerry's criticism of the Vietnam War and his participation in a veterans movement that protested the war. The famous "Swift Boat" commercial featured one woman saying that Kerry gave "aid and comfort to the enemy." When the campaign ended, Kerry's image had been transformed from a decorated military veteran to a New Left hippie who was essentially responsible for threatening to bring down the U.S. government.
In New Hampshire, Gingrich skillfully exposed what line of attack Democrats can take on Romney should he win the nomination. The Democrats will try to tap into the immense anger about the ways in which the economy has left so many Americans behind and rendered families vulnerable to joblessness, bankruptcy and foreclosure.
If Republicans are not careful, Mitt Romney will become the face of a party that voters think doesn't care about ordinary Americans. Former Republican candidate and Arkansas Gov. Mike Huckabee famously said that Romney "looks like the guy who fired you." If Republicans don't figure out a response, by picking up on Huckabee's point, Gingrich has pointed to the criticism that very well might give Democrats another four years in the White House.
Of course, Democrats need to be cautious as well since their new-found strength can also be turned into a weakness. Romney has mounted an aggressive defense of his past by connecting it to a defense of the free enterprise system.
If Democrats push too far, Romney might be able to exploit this criticism by, if he gets the nomination, using it to build on Republican claims that Obama is a "big government liberal" who doesn't respect how markets work. These attacks have been quite effective in the past at painting Obama as left-of-center.
But for now, Romney has a lot of work to do. Newt Gingrich created an opportunity, which Democrats are taking advantage of, to frame Romney as the kind of businessperson who has little concern for jobs, workers or middle-class America.
The opinions expressed in this commentary are solely those of Julian Zelizer.