– The Republican presidential candidates brought increasingly hostile attacks from the campaign trail into prime time on Thursday, clashing over citizenship and conservatism in a contentious debate that reflected the wide-open nature of the nomination battle just weeks before the first votes will be cast.

The first prime-time debate of the new year in some ways bore little resemblance to the five that preceded it. The most obvious difference was the number of candidates participating on the main stage, seven, down from a high of 11.

That smaller stage showcased the dynamic of the race: a tight contest in Iowa where Sen. Ted Cruz's organization and grass-roots appeal have made inroads against Donald Trump's persistent national advantage; a fierce fight between governors looking to position themselves for a strong showing in the New Hampshire primary; and the uncertainty about what happens after that in South Carolina, the site of Thursday's debate.

In the debate's earliest moments, the party hopefuls focused on Democrats: President Obama and front-runner Hillary Clinton. New Jersey Gov. Chris Christie called the president's State of the Union address "story time."

Former Florida Gov. Jeb Bush warned that Clinton, if elected, "might be going back and forth between the White House and the courthouse" amid investigations of her e-mail practices.

But the tenor changed quickly as moderators asked about comments Trump has made in which he questioned whether Cruz, the Texas senator, who was born in Canada, is constitutionally eligible to be president.

Cruz, who has said he is qualified because he is the son of an American citizen, suggested Trump is only raising the issue now because he fears losing the first contest of the season, in Iowa's caucuses on Feb. 1. "I would suggest we focus on who's best prepared to be commander-in-chief, because that's the most important question facing the country," he said.

Trump acknowledged he has raised the issue because Cruz has become a more viable threat, but said it was a legitimate concern. Democrats would probably sue over the issue if Cruz became the nominee, he said. "You can't do that to our party, you really can't," Trump said.

Trump entered the night fresh off an NBC News survey that found he had expanded his lead over the field nationally. One-third of Republican voters favor the real estate mogul, compared with 20 percent for Cruz, 13 percent for Florida Sen. Marco Rubio and 12 percent for Ben Carson.

The billionaire real estate developer has faced his highest-profile attacks to date this week, not only from Cruz but from Obama and South Carolina Republican Gov. Nikki Haley. Earlier this week, giving the GOP response to Obama's State of Union speech, Haley told her own family's immigration story — her parents were born in India — and warned against "the siren call of the angriest voices."

Trump, who initially fired back at Haley in television interviews, said in the debate that he stood by his tone.

"I will gladly accept the mantle of anger," he said. "I'm very angry because our country is being run horribly."

Trump's resilience has baffled and concerned mainstream leaders of both parties, who see his mix of doomsaying and anti-immigrant fervor as dangerous and, in the case of Republicans, potentially damaging to the party's image. Obama devoted much of his speech to an implicit rebuke of Trumpism, defining America as neither in decline nor hostile toward immigrants and Muslims.

Trump and Cruz were at center stage after days of increased sparring. They are as close as they've been in competing for the lead in Iowa, according to polls, and Cruz is in second place in several other states.

Other clashes played out among the other four candidates on the main stage — Rubio, Christie, Bush and Ohio Gov. John Kasich — as they jockey for a strong showing in New Hampshire that could propel them into contention.

Rubio took after Christie, alleging that he had a track record of supporting liberal causes, including Planned Parenthood. Christie retorted that Rubio, who in a previous debate had brushed off attacks from Bush as a sign of weakness, was now doing the same.

Bush, who has often struggled to land attacks on his foes, said the tough talk would continue but stressed the need for the party to ultimately come together.

"Everybody's record is going to be scrutinized," he said. But "at the end of the day we need to unite behind the winner so we can defeat Hillary Clinton, because she is a disaster."

The main debate followed an earlier "undercard" debate by three long-shot candidates, each promising that they'd be the best to take on the Democratic front-runner — glossing over, for the moment, the seven other Republicans they'd have to beat first.

"You cannot wait to see the debate between me and Hillary Clinton. You would pay to see that fight," said former tech executive Carly Fiorina, casting herself as a stand-in for women everywhere.

Former Sen. Rick Santorum and former Arkansas Gov. Mike Huckabee talked about races they'd run against Clinton's allies. "You want a fighter? You want a winner? I'd appreciate your vote," Santorum said.

The Washington Post contributed to this report.