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A few of Romney's rivals are using that ammunition against him, none more aggressively than Newt Gingrich. Starting as early as tomorrow, South Carolina residents can hear about Bain Capital from another source as well, a super-PAC that supports Gingrich but officially isn't part of his presidential campaign. NPR's Peter Overby has more.
PETER OVERBY, BYLINE: The South Carolina primary is a week from Saturday. And before then, voters there can expect to be inundated with ads attacking Romney and Bain Capital.
UNIDENTIFIED MAN #1: Well, we made a $3.4 million ad buy in South Carolina, which is fairly significant.
OVERBY: That's Rick Tyler, senior advisor to the pro-Gingrich super-PAC Winning Our Future, and fairly significant hardly does justice to the super-PAC's plan. If you spent all that money on 30-second spots, the average TV viewer in the Palmetto State would sit through 70 messages like this one - slashing at the legacy of Bain Capital.
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OVERBY: Tyler is a long-time aide to Gingrich. So are others at the helm of Winning Our Future. As a super-PAC, it's supposed to be independent of Gingrich's campaign. But they consciously try to deliver the message he wants voters to hear. In this case, as Tyler puts it...
RICK TYLER: People who think they know Mitt Romney should think again.
OVERBY: From all indications, Gingrich's campaign cannot afford this kind of advertising blitz. And it definitely couldn't take a single contribution of $5 million to pay for one. But that's what Winning Our Future did. The money came from Las Vegas casino magnate Sheldon Adelson. He had already given the legal limit to the campaign. But under the Supreme Court's Citizens United ruling, and other recent changes in the law, he can give as much as he wants to a super-PAC like Winning Our Future.
Adelson made most of his money in casinos. He bought the Sands in Las Vegas, imploded it to rebuild bigger, and now he has huge resorts in Singapore and Macao. In 2010 he told the CNBC show "Managing Asia" that he intends to keep going.
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OVERBY: The growth is not without controversy. The Securities and Exchange Commission and the Justice Department are both investigating allegations of corruption at those developments. Adelson is solidly Republican and generous with his checkbook. He's on the board of the Republican Jewish Coalition and a hardliner on protecting Israel. He's a fan, and a friend, of Newt Gingrich.
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OVERBY: This is a 2009 video for Gingrich's old political organization, American Solutions for Winning the Future.
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OVERBY: Adelson and his wife Miriam were American Solutions' biggest backers, to the tune of nearly $8 million. But with this contribution to the super-PAC, Adelson single-handedly has given Gingrich's presidential bid new life beyond today's New Hampshire primary.
Big donors have always been able to shovel money into presidential elections. But super-PAC's, run by professionals, make it much easier.
BOB BIERSACK: Now all you have to do, as a person with lots of money, is simply write the check.
OVERBY: Bob Biersack is a senior fellow at the nonpartisan Center for Responsive Politics, which tracks political money. He says the unofficial ties between a super-PAC and a candidate make it a straightforward proposition for the donor.
BIERSACK: There isn't much ambiguity about how this money will be used. And while that may or may not have been true in the past, it's certainly true today.
OVERBY: So now Gingrich can hope that his super-PAC will cripple Romney - just the way Romney's super-PAC crippled Gingrich last month.
Peter Overby, NPR News, Washington.