By Samuel P. Jacobs and Alina Selyukh
WASHINGTON (Reuters) - Whenever Republican U.S. presidential candidate Mitt Romney has been down and in need of an enforcer, one has been there.
Restore Our Future, the independent "Super PAC" that supports Romney's campaign, has been a machine of destruction in the race for the Republican presidential nomination, swamping Republican rival Newt Gingrich with attack ads each time he has seemed to threaten Romney's frontrunner status.
Until late on Tuesday, the engines behind the PAC - its donors - were largely secret.
But the group's financial reports to the Federal Election Commission (FEC) reveal that private equity executives, hedge fund managers and other financial heavyweights are key contributors to the fundraising juggernaut that Restore Our Future has become.
Political action committees (PACs) are groups with great clout in U.S. politics that are legally separate from candidates. In this campaign, they have unleashed negative advertising against the rival of the candidate they favor.
The filings reveal that Restore Our Future had $23.6 million in the bank as of December 31 - more than the Romney campaign itself. The PAC has spent $17.4 million in support of Romney, including nearly $11 million in Florida before Romney's decisive victory in the Republican primary there on Tuesday.
The group's filings reveal a broad base of donors who have given $100,000 or more, taking advantage of the lack of limits on donations to such groups. Campaigns are limited to $2,500 donations per donor.
Among the most generous contributors to Romney's cause have been people who share Romney's investment background. Romney, a former Massachusetts governor, co-founded Bain Capital, a private equity firm, and has emphasized his business experience in touting his ability to create jobs and improve the economy.
Three hedge fund managers pitched in $1 million to the pro-Romney PAC: Tiger Management's Julian Robertson, Elliot Management's Paul Singer and Renaissance Technology's Robert Mercer.
Miguel Fernandez, chairman of a healthcare private equity firm, gave $500,000. MBF Family Investments, which shares an address with Fernandez's investment company, gave another $500,000.
The list of major donors to Restore Our Future is studded with other veterans of Wall Street and the investment community.
Tudor Investment Corp. founder Paul Tudor Jones contributed $200,000. Goldman Sachs executive Edward Forst forked over $95,000. Four other Goldman employees gave at least $50,000. Lewis Eisenberg, a leader at private equity firm KKR, gave $25,000.
Romney, mocked by Gingrich as a "Massachusetts moderate," has received the support of conservative money men such as Bob Perry, a Texas home builder, and William Koch, a billionaire investor whose family is active in conservative politics.
"You would think they would be backing much more conservative candidates," said Fred Zeidman, a Houston fundraiser for Romney. "It seems to me what they are saying, 'Hey we've got to beat Barack Obama, and this is the way we are going to do it.'"
Restore Our Future also has drawn contributors from Reebok founder Paul Fireman and philanthropist David Mugar from Boston, Romney's longtime home.
A BROAD BASE OF SUPPORT
Fourteen individuals contributed at least $250,000 to Restore Our Future. That is far more than the number of such donors who have given to competing Super PACs.
Winning Our Future, the group allied with Gingrich, has depended largely on one couple: casino billionaire Sheldon Adelson and his wife, Miriam, who each chipped in $5 million in January, a month after their two daughters had given a total of $750,000.
Our Destiny, a Super PAC that backed former Utah governor Jon Huntsman when he was in the race, raised $2.7 million in the fourth quarter of 2011 - $1.9 million of it from Huntsman's father, industrialist Jon Huntsman Sr.
Wyoming billionaire Foster Friess contributed nearly half the $729,935 raised by the Red White and Blue Fund, which supports another Republican, former Pennsylvania U.S. senator Rick Santorum.
The financial gap between Romney's PAC and the rest of the field is likely to widen, said campaign finance analyst Anthony Corrado.
"I expect it would be much easier for the Romney Super PAC to go out and solicit big contributions after a win in Florida, and the other Super PACs will be even more dependent on the largesse of a few wealthy individuals to continue to be willing to write large checks," said Corrado, a government professor at Colby College in Maine.
Restore Our Future is based in Washington and is run from the law firm Clark Hill by treasurer Charles Spies. It has no offices in any of the 50 states and no staff presence on the campaign trail.
Rivals describe it as a monolithic campaigning machine, with tight-lipped members, lots of money and a favorite target: Gingrich.
"That's a compliment as long as it's an effective machine," said Spies, former counsel for Romney's 2008 campaign.
The group has outspent its rival PACs by far, dumping millions of dollars into TV, Internet, direct-mail and other types of advertisements - nearly all of them attacking Gingrich.
The ads casting Gingrich as an erratic Washington insider with questionable ethics helped send Gingrich's campaign into a dive just before the Iowa caucuses, the first contest in the race to pick a Republican nominee to face Democratic President Barack Obama in the November 6 election.
Restore Our Future's anti-Gingrich ads appear to have had a similar effect in advance of Tuesday's vote in Florida, where the PAC had spent nearly $11 million.
In keeping with Restore Our Future's low profile, spokeswoman Brittany Gross, who fields media requests, declined to comment on the group's scope and strategy.
(Editing by David Lindsey and Will Dunham)
(This story corrects spelling of name in 14th paragraph to Zeidman)