US Republican presidential candidate and former Massachusetts governor Mitt Romney greets supporters at his Florida primary night rally in Tampa, Florida, on Tuesday. [Mike Carlson / Reuters]

WASHINGTON - Groups known as "Super PACs" raised more than $42 million to back Republican US presidential contenders in 2011, according to campaign filings that show how new donation rules are allowing a relatively few wealthy people to shape the race.

The reports filed with the Federal Election Commission (FEC) late on Tuesday offer a vivid picture of the impact of a 2010 US Supreme Court decision that allows unlimited donations to political action committees (PACs), groups that are legally separate from the candidates they support.

The reports showed why the Super PAC supporting Republican front-runner Mitt Romney, called Restore Our Future, has been such a force in the campaign - largely by running attack ads against Newt Gingrich, Romney's top Republican rival.

Restore Our Future hauled in $30 million in 2011, and had nearly $24 million in the bank at the end of the year.

The group spent a big chunk of that during the past month in Florida, where its ad barrage against Gingrich was widely credited with helping Romney to victory in Tuesday's primary. Florida was the latest contest in the state-by-state battle to pick a Republican nominee to challenge Democratic President Barack Obama in the Nov 6 election.

The pro-Romney group's bankroll dwarfed the PACs supporting other Republican contenders, as well as the group that backs Obama. Priorities USA, the pro-Obama group, raised $4.2 million last year and had $1.5 million in the bank on Dec 31.

The funding disparity between the groups suggests the PAC supporting Romney could help the former Massachusetts governor overcome the Obama campaign's formidable fundraising advantage if the two meet in November's general election. Contributions to candidates' campaigns are limited to $2,500 per donor.

Obama's organization continued its dominance in the race for cash among candidates' campaigns, raising $130 million for the year. That topped the Romney campaign's $57 million, which led the Republican presidential field.

Tuesday's filings also revealed the growing war chests that independent Republican groups are building with the presidential and congressional races in mind.

American Crossroads and its affiliated group, Crossroads GPS, raised a total of $51 million in 2011.

Super PACs were forged from the 2010 Supreme Court ruling that erased long-standing limits on corporate and union money in federal elections as an unconstitutional restriction of free speech.

The ruling unleashed a flood of money into a political system coming off the most expensive presidential election in US history in 2008, when candidates spent more than $1 billion. It also opened the door for wealthy individuals to prop up candidates by writing a check.

"Super PACs have fundamentally changed the way campaigns are run, and it's had a huge effect on the race," former Michigan Republican Party chairman Saul Anuzis said. "If you can find one donor who is willing to play in a big way, it can have an unbelievable impact."

For the first time, the FEC reports revealed many of the wealthy donors behind the Super PACs.

Harold Simmons, a billionaire Dallas banker and chairman of Contran Corp, gave American Crossroads $5 million and Gingrich's group $500,000. Contran gave another $2 million to the Crossroads group.

Peter Thiel, billionaire co-founder of the payment service PayPal, gave the Super PAC backing Texas congressman Ron Paul $900,000.

Foster Freiss, a billionaire investor from Wyoming, founded the Red, White and Blue Fund that backs former US senator Rick Santorum and donated $331,000.

Reuters