(CNN) - Having levied his own criticisms of Mitt Romney's background leading the private equity firm Bain Capital, Newt Gingrich has a warning for President Barack Obama's campaign: the issue is fair game, but be wary.
"We found out when we got in a fight with Mitt Romney over this that it didn't work," he said Monday on CNN's "Piers Morgan Tonight," after an Obama surrogate on Sunday called political discourse on both sides of the aisle "nauseating."
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The surrogate, Newark Mayor Cory Booker, called out his own party's criticism of Bain's record under and after Romney's tenure at its helm.
Gingrich said Booker "described what I think is a big reality for him as the Mayor of Newark, that that free enterprise system has been creating jobs, paying taxes, improving his city."
Romney's work at Bain is open for inspection as part of his record, Gingrich said, but Democrats may best stay away from it, lest they be seen as critical of the free market.
"There are things you can legitimately look at in Bain Capital," Gingrich said. "I think there are things you can legitimately look at in anyone's record, including Mitt Romney's record."
The former House speaker made Romney's record with the firm central to his campaign's criticism of the former Massachusetts governor, and the super PAC supporting him featured it prominently in television advertisements.
"I would just say that if Governor Romney would like to give back all of the money he's earned from bankrupting companies and laying off employees over his years at Bain, then I would be glad to listen to him," Gingrich said while campaigning.
Gingrich, who now supports Romney as the alternative to reelecting Obama, said he sees no hypocrisy in his support of Romney. In his interview with CNN's Piers Morgan, he primarily stuck to his own political analysis of Bain-related criticisms of Romney.
"One thing we discovered we could never make clear an attack on a particular case, and Romney's ability to say no this is about free enterprise" he said. "And the average American looked up and said, 'it's about free enterprise.' And it turned out that particular argument simply doesn't work."