Adding to its cherished status as a presidential proving ground, Iowa is shaping up as a harbinger of the leading role that independent groups will play in the 2012 campaigns.

Iowans were inundated with millions of dollars in negative advertising in the final weeks before Tuesday’s caucuses, most of it paid for by a new breed of organization, called “super PACs,” which don’t have to play by the same rules as candidates.

Super PACs have outspent Republican candidates by more than 2 to 1 in Iowa and other early primary states this election cycle, according to data from the Federal Election Commission and Kantar Media/CMAG, which tracks ad spending. The gulf is even wider when the picture is broadened to include other independent groups, many of which already haves spots on the air in Colorado, Ohio and states that are likely to be key battlegrounds in November.

The clearest beneficiary of the onslaught has been former Massachusetts governor Mitt Romney, who was able to fend off a last-minute surge by former House speaker Newt Gingrich (Ga.) with the help of more than $4 million in advertising from Restore Our Future, a super PAC staffed by former Romney advisers. Romney’s campaign spent relatively little ad money in Iowa.

The trend marks a major shift from the 2008 election cycle, when outside groups were subject to more legal restrictions and played a relatively minor role in the presidential contest. But after a series of court rulings in 2010, super PACs and other independent organizations are free to raise unlimited amounts of money from corporations and wealthy individuals — significantly increasing their ability to influence the GOP primaries as well as the general election.

Super PACs, unlike traditional political action committees, are not allowed to coordinate directly with candidates. That allows the contenders to distance themselves from attacks that have become the norm for many super PACs.

“You’re going to see outside groups play a much bigger role than they did in 2008 or even 2004,” said Carl Forti, a Restore Our Future adviser and political director at American Crossroads, a pro-Republican group that says it plans to raise $240 million in the 2012 cycle. “The landscape has changed.”

Crossroads and other independent groups have already set their sights on the next primary states, spending $1.5 million on advertising targeting New Hampshire, about $1 million in South Carolina and nearly $5 million in Florida, according to Kantar’s latest data.

The early spending hasn’t been confined to primary states, either. In the bellwether state of Ohio, Sen. Sherrod Brown (D) has been the target of nearly $3 million worth of attack ads from the U.S. Chamber of Commerce, a Crossroads nonprofit arm and other Republican-leaning groups, according to statistics tallied by Democrats.

Neither Brown nor Ohio State Treasurer Josh Mandel, his GOP challenger, have aired any of their own ads yet. Justin Barasky, a spokesman for the Ohio Democratic Party, said the influx of outside money is likely to make the contest much closer than it would be otherwise.

“Three million dollars is a lot of money in a Senate race, and they obviously have a lot more,” Barasky said.

The top super-PAC spenders are Restore Our Future, with $4.1 million; Make Us Great Again, which supports Texas Gov. Rick Perry, at $3.8 million; and Our Destiny PAC, backing former Utah governor Jon Huntsman Jr., with $2.2 million, according to FEC data. Two super PACs supporting former senator Rick Santorum (Pa.), a late-surging candidate in Iowa, have spent about $600,000.

In Iowa, Restore Our Future announced a burst of ads targeting Gingrich last month after he vaulted to the top of many polls. The group, which was started earlier this year by Forti and two other former Romney aides, reported raising $12 million through June.

Under Federal Election Commission rules, Restore Our Future does not have to report any more of its donors until Jan. 31. And despite the prohibition against coordination with candidates, super PACs can enlist the help of candidates to raise money.

A pro-Gingrich super PAC called Winning Our Future has spent $800,000 on ads, FEC records show. Gingrich criticized Romney on Tuesday for his connections to Restore Our Future.

“This is a man whose staff created the PAC, his millionaire friends fund the PAC, he pretends he has nothing to do with the PAC — it’s baloney,” Gingrich told CBS News.

The Romney campaign did not respond to Gingrich’s remarks.

If a candidate drops out, a super PAC formed to support him or her could, among other things, dispense its money to other groups or switch allegiances. One super PAC that had said it was backing Bachmann, Citizens for a Working America, bought $475,000 worth of advertising in support of Romney in Iowa last week.

Democrats have had little success in matching GOP-allied groups in fundraising, leaving most of the messaging in favor of President Obama to the Democratic National Committee, which is subject to contribution limits. One pro-Obama group, Priorities USA, which said it raised about $5 million through June, has spent only about $700,000 on advertising.

Priorities spokesman Bill Burton said the group will use social media and other unconventional strategies to counter better-funded groups on the right.

“I think that the real lesson of December is that Mitt Romney has access to tens of millions of dollars in support from outside groups, and they’re going to put it to work,” Burton said.

Staff writer T.W. Farnam contributed to this report.