WASHINGTON (AP) — Think of super PACs as shadow cash machines for presidential candidates. They're going to be big this year.

Real big.

The independent fundraising groups can gather and spend unlimited money to run ads supporting a candidate or attacking a rival. The leading Republican contenders, Texas Gov. Rick Perry and former Massachusetts Gov. Mitt Romney, both have at least one super PAC working to boost their candidacies. Another super PAC is backing President Barack Obama's re-election bid.

Although they aren't permitted to coordinate directly with the campaigns, which must follow strict federal restrictions on what they can raise and spend, many of the groups are staffed by former aides and fundraisers who know the candidates' thinking and strategy. Watchdog groups see super PACs as just the latest erosion of campaign finance rules that date back to the Watergate era of the 1970s.

Republican-leaning super PACs were first influential in the 2010 congressional elections. Now, presidential contenders are receiving millions of dollars in financial backing from new free-spending, unregulated political action groups.

Make Us Great Again PAC, a super PAC supporting Republican front-runner Rick Perry, was co-founded by Mike Toomey, a former chief of staff to the Texas governor. Documents show the group plans to spend $55 million to support Perry's White House run.

The Perry-aligned super PAC will have to compete with Restore Our Future, formed to boost his top rival, Mitt Romney. It raised $20 million from January through June. Its treasurer, Charles Spies, was general counsel for Romney's 2008 White House bid.

The third-quarter 2011 campaign fundraising period ends Friday. Campaign finance reports from that quarter will offer a clearer picture of how much help the super PACs will be able to offer their candidates.

The super PACs that favor certain candidates have already begun spending on television advertising. Keeping Conservatives United, a super PAC supporting Minnesota Rep. Michele Bachmann, assailed Perry's record in Texas.

Candidates will continue to do their own campaign fundraising, following much more restrictive federal rules than those governing super PACs. A presidential campaign can raise at most $5,000 total from an individual donor — $2,500 each for the primary and general elections. Super PACs can solicit and spend unlimited funds. Some super PACs also have affiliated groups whose donors are allowed to remain anonymous.

One of those is Priorities USA, a super PAC formed by former Obama White House aides Bill Burton and Sean Sweeney to promote the president's re-election. A sister group, Priorities USA Action, allows anonymous contributions even though the president has publicly criticized them.

Obama raised $750 million for his 2008 campaign and is looking to exceed that total for his re-election effort, raising the question of why he would even need a super PAC working on his behalf. And the Perry and Romney campaigns are expected to far outpace all other GOP competitors in fundraising, with their aligned super PACs only helping to cement that advantage.

To some degree, outside groups are supplanting the role of the major political parties, which help coordinate messages and get-out-the-vote efforts for candidates, but also operate under strict federal fundraising rules. The party committees remain an important, if somewhat diluted, force. The Democratic National Committee has raised more than $80 million to support Obama's re-election campaign, recent federal filings show.

After the Watergate scandal, Congress put limits on campaign fundraising and spending as a way to stem the corruptive influence of money and created the Federal Election Commission to enforce those limits. Government watchdogs say super PACs are undermining that intent.

"The most dangerous vehicle for corruption in the political system is the candidate-specific super PAC," said Democracy 21 president Fred Wertheimer, a longtime advocate for campaign-finance limits. "If we can't shut them down, they are going to spread like wildfire to members of Congress, and they're going to eviscerate contribution limits enacted over a period of a century to prevent corruption of federal officeholders."

Restore Our Future, which backs Romney, came under scrutiny recently for a $1 million contribution it received from a donor identified as W Spann LLC — actually a corporation set up by Edward Conard, a former Romney colleague at the Bain Capital investment firm. Restore Our Future's report to the Federal Election Commission was amended once Conard stepped forward, and Romney has brushed off the controversy as a nonissue.

Romney spoke at a July fundraiser for Restore Our Future, raising questions of how closely a candidate can work with a super PAC before crossing the line into coordination.

Some Washington campaign veterans aren't convinced the new PACs will by default be corruptive. Michael Toner, counsel to Minnesota Gov. Tim Pawlenty's now-abandoned presidential campaign, said the outside groups will bring a host of new messages — competing in the marketplace of ideas — that are transparent in their sources of funding.

Super PACs made their debut following a Supreme Court decision that removed restrictions on corporate and union spending in elections. Super PACs spent more than $65 million that cycle, according to data from the nonpartisan Center for Responsive Politics, which tracks campaign spending. Republican-leaning groups far outpaced their Democratic counterparts, helping Republicans win control of the House and pick up six seats in the Senate.

The biggest-spending super PAC that year was American Crossroads, a GOP group connected to Karl Rove, former President George W. Bush's longtime political director. Crossroads and an affiliated organization, Crossroads GPS, spent nearly $38 million to influence House and Senate races.

Crossroads has already raised $25 million this year and has spent more than $1 million on ads in several special elections around the country, as well as to influence the debate over Obama's new $447 million jobs proposal and the negotiations last summer over raising the federal debt ceiling. Crossroads president Steven Law said super PACs help give voters an abundance of competitive information — not just from well-funded incumbents — that's ultimately good for the country.

"Just with President Obama's billion-dollar campaign and the union juggernaut alone, we believe that Republicans will be significantly outspent in 2012," he said, "and our goal is to prevent them from using that crushing weight to irradiate Republican candidates up and down the ticket."

Democrats say they formed their own super PACs as a way to respond to the huge Republican fundraising advantage in 2010.

"You play with the rules that you're given, and you cede no ground," said Rodell Mollineau, the president of American Bridge 21st Century, a research organization for Democratic super PACs. "You have to push back forcefully."