WASHINGTON - Presidential candidate Newt Gingrich appeared at ease in his new role as the Republican front-runner to challenge President Barack Obama in 2012, handily brushing aside criticism levelled at him in the 12th debate among party hopefuls.

In the Saturday night session in Des Moines, Iowa, all the candidates attacked Obama at his most vulnerable spot -- the struggling U.S. economy -- but seemed even more eager for the chance to undermine Gingrich, who has shot past former Massachusetts Gov. Mitt Romney who had been coasting at the top of nationwide polls of Republicans for months.

He has lost that spot with less than a month remaining before the Iowa precinct caucuses on Jan. 3 begin the process of selecting delegates who will choose the nominee at the Republican National Convention next summer.

In perhaps the most rigorous exchange, Gingrich, the former speaker of the House of Representatives, dismissed criticism of his assertion a day earlier that the Palestinians were an "invented" people. He has been roundly charged with making the stalled Mideast peace process even more difficult.

"Is what I said factually correct? Yes. Is it historically true? Yes," said Gingrich, a former college history professor, drawing applause as he said it was time someone spoke the truth about the nature of Israel's struggle with the Palestinians.

"Somebody ought to have the courage to tell the truth. These people are terrorists," he said. "It's fundamentally time for somebody to have the guts to stand up and say, 'Enough lying about the Middle East."'

Gingrich's remarks were an opening for Romney to press his effort to portray Gingrich as too erratic and bombastic to be president.

It was "a mistake on the speaker's part," Romney said. "We're going to tell the truth, but we're not going to throw incendiary words into a place which is a boiling pot when our friends, the Israelis, would probably say, 'What in the world are you doing?"'

"I'm not a bomb thrower, rhetorically or literally," Romney said, recalling Gingrich's reputation from his days as leader of the House, a position he resigned from after being fined $300,000 for ethics violations.

That was only one of the tough exchanges. Romney also has been running TV ads in Iowa -- playing on voters' disgust with the government in Washington -- that label Gingrich as a career politicians.

That gave Gingrich the opening to fire back: "The only reason you didn't become a career politician is because you lost to Teddy Kennedy in 1994." He was referring to Romney's failed bid to unseat Kennedy from his Massachusetts Senate seat.

"That's probably true," replied Romney, who then quipped that if he'd achieved his childhood dream, "I would have been a football star all my life, too."

Gingrich's decision to invoke Kennedy, the late senator, served as a dual reminder -- that Romney has been running for office since the mid-1990s and that in his Massachusetts campaigns Romney often took more moderate positions on such issues as abortion and gay rights than in his campaigns for the Republican presidential nomination in 2008 and now. Gingrich has accused Romney of running to the left of Kennedy whose politics conservatives detested above all others.

Both Texas Gov. Rick Perry and Minnesota Representative Michele Bachmann tried to tie Romney and Gingrich together, declaring neither man was a true conservative. Both Romney and Gingrich still face skepticism from conservative Republicans, especially those in the tea party movement.

Bachmann referred repeatedly to "Newt-Romney," saying the two hold similar views on health care reform, illegal immigration, climate change legislation and the payroll tax cut extension that Obama is trying to push through the Republican-controlled House before it lapses at year's end.

Romney, Gingrich and Texas Rep. Ron Paul said they favoured the payroll tax cut extension. Bachmann, Perry and former Pennsylvania Sen. Rick Santorum said they opposed the measure.

The debate was the first since former businessman Herman Cain left the race after allegations surfaced of sexual harassment and an extramarital affair.

Gingrich's personal life also is a campaign issue, and it came up Saturday night.

Asked whether marital fidelity was a valid concern for voters choosing a candidate, Perry said, "If you cheat on your wife, you'll cheat on your business partner. It's a characteristic people look at."

The thrice-married Gingrich, who has been divorced twice and has admitted past infidelity, said the issue was an important one, then added, "I've made mistakes at times and I've had to go to God for forgiveness."

The libertarian-leaning Paul, who is faring well in polls of likely Iowa caucus-goers, has been airing television commercials there attacking Gingrich. Romney's campaign has become increasingly critical of Gingrich as well, bolstered by a multimillion-dollar television ad campaign that is financed by allies. Under questioning from Paul, Gingrich said he had never lobbied for Freddie Mac, the quasi-government agency that guarantees home mortgages, that paid him at least $1.6 million to provide strategic advice. Paul shot back, "It's the taxpayers' money, though. We were bailing them out."

Only six of seven candidates were at the faceoff. Jon Huntsman, the moderate former Utah governor and Obama's first ambassador to China, was absent in deeply conservative Iowa, where he has no chance of winning. He has staked his hopes on a strong showing in New Hampshire, which holds the first presidential primary on Jan. 10.