The blast of negative ads that helped torpedo Newt Gingrich ahead of Tuesday's Iowa caucuses are just a small splatter of the mud set to fly when the eventual Republican nominee goes up against President Barack Obama.

"It's going to be a very expensive year with a lot of negative advertising," predicted Steffen Schmidt, a political science professor at Iowa State University.

Negative ads have long played a role in US politics, but this will be the first presidential campaign in which independent groups can join the fray free from previously strict restrictions.

The new rules have allowed Mitt Romney to keep his hands relatively clean while the Restore our Future -- a super PAC, or political action committee -- filled Iowa mailboxes and airwaves with ads attacking Gingrich.

Backed by a slew of negative stories in the media and opposition from the Republican establishment ahead of the party's first nominating event, the ads have worked.

Gingrich, the former US House speaker, saw his brief surge rapidly slip to around 13 percent after polling in the low 30s in mid-November and earlier this month.

"Today's polls reflect the devastation inflicted on Gingrich by pro-Romney Super PAC, which has nuked Newt with millions in negative ads," top Obama adviser David Axelrod tweeted Wednesday.

Gingrich has chastised his rivals for the negative ads, but Romney responded by saying it's just "part of the process" and good preparation for a battle with Obama.

Federal election laws used to make it a crime for independent groups to run ads that urged voters to support or defeat a specific candidate and also restricted heavily independent advertising about election-related issues in the weeks leading up to an election or primary.

That changed when the Supreme Court's landmark Citizens United ruling in January 2010 unleashed a flood of money into the election process that isn't affected by the disclosure rules and caps applied to donations made directly to candidates and political parties.

The degree and intensity of the negative onslaught has intensified dramatically, said Mark Lundberg, chairman of the Republican party for Sioux County, Iowa.

"I've been riding this rodeo since 1976. I don't know if there's any more ads this time than before, but the negative tone is much larger than it has been in the past," he told AFP.

Restore Our Future has run five ads attacking Gingrich for supporting so-called amnesty for illegal immigrants, as well as for teaming up with Democratic House minority leader Nancy Pelosi on global warming and ethics violations.

"Know what makes Barack Obama Happy? Newt Gingrich's baggage," one ad warned.

A Christian evangelical group called Illuminati released even nastier ads, calling Gingrich the "walking, talking definition of untrustworthy" and attacking him for his marital infidelities.

That same group has also attacked Romney, saying he is a "liberal monster" who "instituted $50 state-funded abortion" while governor of Massachusetts and is "even worse than Barack Obama" because he will usher in the "slow death" of the conservative movement.

But nearly all the money spent by outside groups opposing candidates was aimed at Gingrich, who accounted for $2.7 million compared with just $330,000 spent opposing Romney in the latest tally by the Center for Responsive Politics.

Gingrich isn't the only Republican worried that these ads will provide ample fodder for Obama, who had raised nearly as much as the nine leading Republican contenders combined at the end of the last reporting period and won't have to spend anything on a contentious primary campaign.

"It always benefits the incumbent when the potential opponents are going at each other like this," Lundberg said.

Incredible amounts of resources and efforts are being spent that could have been saved up for the general election, helping fuel rifts among the party faithful.

The negative primary ads can upset people enough that they won't vote for the Republican nominee in November.

"That's a relatively small number of people, but every vote counts," Lundberg said.

Not everyone thinks that the unrestricted spending by independent groups or negative advertising is a bad thing.

"We have operated as a nation for over 200 years on the theory that the more speech that's out there the better and that people can act on the information they receive," said Jan Baran, a campaign finance attorney with Wiley Rein LLP in Washington.

"The spending of money in a campaign is part of that very expensive conversation that goes on in elections."