DES MOINES, Iowa - Newt Gingrich defended himself in his first debate as the new front-runner for the Republican presidential nomination Saturday, with less than a month before the leadoff vote to determine President Barack Obama's challenger in 2012.

The former speaker of the House of Representatives has risen to become the new target in the race, and former Massachusetts Governor Mitt Romney has targeted him directly in recent days after leading in earlier national polls.

The candidates were debating in Iowa, where precincts caucuses on Jan. 3 begin the process of selecting delegates to the Republican National Convention next summer.

The night's topics focused on domestic instead of foreign affairs, but Gingrich was again questioned about his comments the day before that Palestinians are an "invented" people. Gingrich tried to clarify that before the debate, saying he supports a negotiated peace agreement between Israel and the Palestinians, including a Palestinian state.

Romney told the debate that Gingrich has made it more difficult for Israelis and Palestinians to negotiate toward peace.

Gingrich responded by saying he is speaking as a historian but added that it's time for a candidate to stand up and call Palestinian leaders "terrorists."

On the domestic front, Romney again emphasized his experience as a businessman in the private sector and suggested that Gingrich's background as a Washington insider would be a liability.

Gingrich fought back, telling Romney, "The only reason you didn't become a career politician is because you lost to Teddy Kennedy in 1994" in a U.S. Senate bid.

Romney quickly retorted: "If I'd been able to get in the NFL as I'd hoped as a kid, I could have been a football star 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 also that he lost to the man whose politics conservatives detested above all others.

For his part, Texas Governor Rick Perry made a remark aimed at Gingrich, who has admitted past infidelity, saying, "If you cheat on your wife, you'll cheat on your business partner. It's a characteristic people look at."

Both Perry and Minnesota Representative Michele Bachmann tried to tie Romney and Gingrich together, calling them not conservative. Both Romney and Gingrich still face skepticism from conservative tea party movement.

Bachmann referred repeatedly to "Newt-Romney," saying the two hold similar views on health care, illegal immigration, cap-and-trade legislation and the payroll tax cut extension.

But all candidates agreed they must attack unemployment and revitalize an anemic economy, which looms as the top issue in the election.

The debate was the 12th debate of the Republican nominating campaign and the first since Herman Cain suspended his campaign a week ago amid allegations of sexual impropriety.

Romney has turned in a series of strong debate performances. But it's Gingrich's performances that voters have noticed.

In a Des Moines Register poll released in early December, 50 per cent of likely Iowa caucus-goers said Gingrich is the best debater. Romney was a distant second with 14 per cent.