A group of prominent Mitt Romney backers has quietly started a “super PAC” to raise and spend unlimited amounts of money in support of his bid for the Republican presidential nomination, according to organizers and others involved in the effort.

Restore Our Future PAC, spearheaded by several former Romney aides, is the latest in an expanding list of groups that have formed to take advantage of court rulings that allow corporations, unions and tycoons to spend millions on elections without restrictions.

The development provides further evidence that Romney, a former Massachusetts governor, is shaping up as the candidate to beat in the GOP money race, and underscores the extensive role that well-funded outside groups are likely to play in the 2012 election. Restore Our Future PAC appears to be the first super PAC to form specifically in favor of one of the presidential contenders.

The group aims to go head-to-head with Priorities USA Action, a similar super PAC led by former White House aides in support of President Obama. But Restore Our Future PAC could be used to go after other Republicans if necessary in the party’s primary season, those familiar with the operation said.

The new super PAC has been soliciting donations in recent weeks, but organizers declined to say how much has been raised.

“This is an independent effort focused on getting Romney elected president,” said Charles R. Spies, the super PAC’s treasurer, who served as Romney’s general counsel in his 2008 White House bid. “We will do that by focusing on jobs and his ability to fix the economy.”

In its Federal Election Commission filing next month, the campaign — which is barred from coordinating with Restore Our Future PAC and any other outside group — could show that it has raised $20 million in contributions, a campaign aide said.

“We just want to show that we’ve got more dough than anyone,” said one major Romney donor, who spoke on the condition of anonymity to discuss internal deliberations.

Gail Gitcho, the Romney campaign’s communications director, said outside support is welcome, given the existence of Democratic super PACs and predictions that the Obama campaign could raise as much as $1 billion for his reelection bid.

“We are pleased that independent groups will be active in fighting this entrenched power so the country can get back to work,” Gitcho said.

Restore Our Future PAC’s board of directors includes Spies; Carl Forti, political director for Romney’s 2008 campaign; and Larry McCarthy, a member of the Romney media team in 2008.

Forti, co-founder of the Black Rock Group consulting firm, also serves as political director for American Crossroads, a separate super PAC that plans to raise at least $120 million for 2012. Forti and American Crossroads did not respond to requests for comment Thursday.

Romney supporters affiliated with the new organization have been briefing top donors in recent weeks about the super PAC’s plans and fundraising goals, including small-group meetings and one-on-one dinners, sources said.

Restore Our Future PAC registered its name with the FEC in October but did not indicate its plans to support Romney and largely remained dormant until recent weeks. It will report its first fundraising numbers next month.

“I’m not surprised that there’s even more money coming into this race to help Mitt Romney,” said Bill Burton, a former Obama spokesman who co-founded Priorities USA Action, which sponsored ads attacking Romney last month in South Carolina. “He’s a pretty deeply flawed candidate; he’s going to need all the help he can get.”

In addition to Priorities USA, Democratic operatives have started two other super PACs to help House and Senate candidates. The groups are enlisting the aid of Democratic leaders to raise money.

This week, Senate Majority Leader Harry M. Reid (D-Nev.) sent out a letter on behalf of Majority PAC, which will focus on Senate races and includes several former top Reid aides.

“There can be no greater priority than protecting the Democratic majority in the Senate,” Reid wrote in a fundraising e-mail, which was first reported by the Center for Responsive Politics. “I’m writing to introduce you to a group solely devoted to leveling the playing field and protecting the Democratic majority in the Senate: Majority PAC.”

Reid asked donors for as much as $5,000, the legal limit for a political committee under FEC rules. But the super PAC will be free to follow up with donors on its own, asking for much larger, and unlimited, contributions, campaign finance experts said.

“The outside groups are akin to the biggest booster club you can imagine for a college football team,” said Dave Levinthal, spokesman for the Center for Responsive Politics, which analyzes campaign-finance data. “The club can’t give cars or gifts to the players, but they can do everything else possible to support them. . . . It’s a brand-new way to play politics.”