Mitt Romney refused to be pinned down Wednesday on how Congress should break an impasse that threatens to raise taxes for 160 million workers _ the latest pressing policy debate the Republican presidential hopeful has sidestepped. Rival Newt Gingrich, in contrast, castigated Congress for "an absurd dereliction of duty."

With less than two weeks before the Iowa caucuses, the two rivals exchanged ever sharper words over a barrage of negative ads coming from Romney's allies as they took a divergent approach on the payroll tax dispute deadlocking Washington.

"I'm not going to get into the back-and-forth on the congressional sausage-making process," Romney, the former Massachusetts governor, said in Keene, N.H., as the day began. "I hope they're able to sit down and work out a solution that works for the American people. My hope is that the solution includes extension of the payroll tax holiday."

But Romney left open the terms for an extension, which is the crux of the stalled debate in Washington. He suggested the extension should last more than two months and ideally a year, but called such details "deep in the weeds."

House Republicans rejected a bipartisan compromise in the Senate that would have kept the tax cuts going for two months, instead calling for talks seeking a one-year extension. The cuts expire Jan. 1 unless Congress acts, and there is little chance of achieving that in time to head off a hit in people's paychecks.

In Iowa, Gingrich called a two-month extension "insufficient" and scolded the Democratic-controlled Senate, Majority Leader Harry Reid and President Barack Obama's administration for "lurching from failure to failure" and marveled: "They can't figure out how to pass a one-year extension, so the Senate leaves town?"

"It's game-playing," added the former House speaker, who stopped short of criticizing House Republicans and their leader Ohio Rep. John Boehner. Gingrich also did not criticize Sen. Mitch McConnell, the Republican Senate leader from Kentucky who signed off on the short-term extension.

The different postures over the payroll tax extension played out against a backdrop of intensifying rancor _ and a dispute over negative advertising _ between Romney and Gingrich with the Jan. 3 Iowa caucuses drawing close.

Romney argued that Gingrich wasn't strong enough to withstand the criticism coming his way, especially from $2.8 million in ads by the pro-Romney group Restore Our Future. The group is going after Gingrich relentlessly in Iowa and exacting a price in the former speaker's standing in polls.

"I'm sure I could go out and say, `Please, don't do anything negative,'" Romney said on Fox News. "But this is politics. And if you can't stand the heat in this little kitchen, wait until Obama's Hell's Kitchen turns up the heat."

Gingrich shot back from Manchester, N.H.: "If he wants to test the heat, I'll meet him anywhere in Iowa next week." He went on: "If he wants to try out the kitchen, I'll be glad to debate him anywhere. We'll bring his ads and he can defend them."

Gingrich tried to show he was aiming for a higher road. He started collecting petition signatures from like-minded people who don't want to see the Republican candidates ripping into each other.

"Attacking fellow Republicans only helps one person: Barack Obama," the petition states.

Gingrich has complained that the Restore Our Future ads, most painting him as an ethically challenged Washington power broker, are untrue. But he declined to say during a news conference in Des Moines on Wednesday what specifically is inaccurate about the ads, instead citing independent reviews that have questioned their validity.

"It would be nice if Governor Romney was either honest about his former staff and his supporters running negative ads, and either disown them _ ask that they take them off the air _ or admit this is his campaign," Gingrich said.

For his part, Romney largely took a pass on the payroll tax matter despite casting himself as an outsider with the business expertise necessary to fix Washington and the economy. He has spent much of the year declining to weigh in on the hot-button fiscal issues Congress has wrestled over.

He stayed out of the summertime fight over raising the federal debt ceiling, urging cooperation but stopping short of endorsing the House GOP's one-year extension or the Senate's two-month extension. He eventually opposed the deal.

In the spring, he was initially reluctant to embrace Wisconsin Rep. Paul Ryan's budget proposal, which would have essentially transformed Medicare into a voucher system. Since then, he's endorsed parts of it.

And on Wednesday, Romney told NBC's Chuck Todd that if he becomes the nominee, he will not release his tax returns to the public.

Gingrich, conversely, hasn't shied away from injecting himself into the latest debates on Capitol Hill, eager to show that he has the leadership qualifications necessary to run Washington and the country _ even when it was politically perilous.

He supported raising the debt ceiling, anathema to many conservatives, and castigated the Medicare portion of the Ryan plan, popular with the GOP's right flank, as "right-wing social engineering," a phrase he later apologized for using. On the latest issue, Gingrich favors a one-year extension of the payroll tax cut.

He has argued that he's a proven national leader for having battled Democratic President Bill Clinton in the 1990s. He usually doesn't mention that as House speaker back then, he bore much of the blame for the federal government twice shutting down when he could not agree on a budget with the Clinton White House.

On Wednesday, in Iowa, he tried to use the latest stalemate to his advantage, saying: "This is an example of why people are sick of Washington and sick of politics."

Meanwhile, in eastern Iowa, Minnesota Rep. Michele Bachmann, vying for the conservative crown in Iowa, blasted the extension as "a very bad proposition" that's too costly.

Gingrich is trying to end his slide in Iowa, where the attacks have taken hold in the past two weeks, with a show of force from establishment Republican leaders in early voting states endorsing his candidacy. He also was dispatching former Oklahoma Rep. J.C. Watts to Iowa to meet privately with GOP leaders and the media in an attempt to stabilize the campaign.

The spat between Romney and Gingrich over third-party ads has dominated the GOP campaign in recent days and highlighted the role of super PACs, independent groups that may accept unlimited donations but are not supposed to directly coordinate with candidates.

Such groups have sprung up to support every serious Republican candidate following a Supreme Court decision last year that said individuals, unions and corporations can donate unlimited sums of money to outfits advocating the election or defeat of candidates.

Two pro-Gingrich groups have started raising money and Gingrich's longtime aide Rick Tyler just signed on with one of them. But Romney's supporters have had a head start in raising money and are slated to spend $3 million this month in Iowa alone, most on anti-Gingrich ads.

Gingrich, who trails Romney badly in fundraising, said he would disavow any group that runs negative ads on his behalf.

Signaling a possible softening of ads just before the holiday, Gingrich this week began showing a Christmas greeting commercial while Texas Gov. Rick Perry launched one featuring his wife, Anita.

But the spirit hadn't hit with a group supporting Perry, which launched an ad criticizing Romney for his past support of abortion rights and Gingrich for ethics violations while in Congress.

In a sign of his fundraising and organizational deficiencies, Gingrich was rushing home to Virginia later Wednesday to help ensure he has the needed signatures to get on the ballot there.

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Hunt reported from Keene, N.H. Associated Press writer Steve Peoples in Manchester, N.H., contributed to this report.