Oskaloosa, Iowa, Dec 21: The fight between Republican presidential front-runners Mitt Romney and Newt Gingrich is escalating, with independently funded attack ads dominating the volatile contest to challenge President Barack Obama in the November election.

Iowa casts the first votes in the battle for the party's nomination on Jan. 3, and the results will help shape the state-by-state march to the Republican nominating convention in August.

After soaring to the top of the polls both in Iowa and nationally in recent weeks, former House of Representatives speaker Gingrich finds himself struggling under more than $1 million in negative advertising, much of it paid for by supporters of Romney, the former Massachusetts governor.

Both men criticize a new campaign finance system — fueled by deep-pocketed political action committees — that benefits them.

In Iowa, Gingrich vowed his White House bid would remain positive but called the Romney camp's tactics "disgusting."

Gingrich, known for a bare-knuckles campaign style when he engineered the Republican takeover of the House in the 1990s, suggested at a campaign stop on Tuesday that his opponents "hire consultants who get drunk, sit around and write stupid ads."

Romney told Fox News on Wednesday that if Gingrich can't take the heat generated by attack ads, he'll wilt during a one-on-one contest against Obama.

On Tuesday, Romney told MSNBC that super PACs have been "a disaster."

But he refused to urge the group Restore Our Future to halt the attacks on Gingrich, saying the law prohibits coordination between his campaign and such groups.

He pointedly declined to disavow the ads.

"I'm not allowed to communicate with a super PAC in any way, shape or form," said Romney, who has been campaigning in the Jan. 10-voting state of New Hampshire.

Gingrich later read Romney's remarks to reporters and said, "His comments are palpably misleading, clearly false and are politics at its worst form."

Meanwhile, Ron Paul has emerged as a wild card in the race. The blunt-spoken Texas congressman will campaign in Iowa again on Wednesday.

The race remains unpredictable, as voters weigh electability against conservative credentials.

An earlier Associated Press-GfK poll found Romney and Obama essentially tied when Americans were asked their voting intentions for 2012.

But Obama led Gingrich, 51 percent to 42 percent.

The bickering over negative ads has highlighted the role of so-called super PACs, independent groups which may accept unlimited donations but are not supposed to directly coordinate with candidates.

Such groups have sprung up to work on behalf of every serious Republican candidate in the wake of a U.S. Supreme Court decision last year that allowed people, unions and corporations to donate unlimited sums of money to outfits advocating the election or defeat of candidates.

Both Romney and Gingrich have PACs leveraging dollars on their behalf.

Among the most visible is Restore our Future, run by former Romney aides.

It has been blanketing the airwaves in Iowa with a series of caustic ads painting Gingrich as an ethically challenged Washington power broker.

The onslaught has chipped away at Gingrich's poll numbers in the state.

Gingrich has challenged Romney to demand the ads come down.

Gingrich trails Romney badly in fundraising, and his campaign had been deeply in debt following staff upheaval earlier this year.

The PAC spending could help him rapidly make up that gap. Gingrich has said he would disavow any PAC that runs negative ads, but that doesn't mean they could not do so anyway.

Gingrich and Romney were each expected back on the campaign trail Wednesday, one of the last full days before the campaigns go quiet for the holidays.

Romney had a full day of campaign stops in New Hampshire, a must-win state for him.