NORTH CHARLESTON, S.C. (AP) — The rivalry between the Republican Party's two leading candidates for president intensified in Thursday's debate, which featured the most rollicking action to date.

Yet rising tensions between billionaire businessman Donald Trump and Texas Sen. Ted Cruz left little space for any of the other candidates to make an impression.

With less than three weeks until Iowa's leadoff caucuses, the renewed focus on the two candidates leading most preference polls suggests the overall shape of the 2016 contest may be solidifying — much to the dismay of Republican officials who fear neither Trump nor Cruz is electable in a general election.

Trump and Cruz starred in Thursday's primetime event. And Trump, in particular, may have had his finest debate performance.

Florida Sen. Marco Rubio and New Jersey Gov. Chris Christie had strong moments, but didn't have the breakout needed to begin coalescing mainstream Republicans behind their candidacies.

Just one debate remains before Iowa's Feb. 1 caucuses. But the likelihood of a major shakeup in the race becomes less and less as days pass.

Here are some other key takeaways from the Thursday night debate.

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NEW YORK, NEW YORK

Cruz and Trump went toe-to-toe several times, including Trump's questioning of whether Cruz, born in Canada to a U.S. citizen, is eligible to serve as president.

Yet no exchange encapsulated the night better than their clash over "New York values."

"Everyone understands that the values in New York City are socially liberal ... focused around money and the media," Cruz charged. And in a nod to Trump's home in midtown Manhattan, Cruz said: "I can frame it another way: Not a lot of conservatives come out of Manhattan. I'm just saying."

Just as Cruz was ready to dismiss concerns about his Canadian birth, Trump was ready to defend New York.

The real-estate mogul said he found that kind of talk "insulting." And he seemed to show his softer side while reflecting on how New Yorkers came together after the Sept. 11 terrorist attacks. "Everybody in the world watched, and everyone in the world loved New York and loved New Yorkers," he said. "That was a very insulting statement that Ted made."

It was hard to imagine a stronger response from Trump, who puts a lot less time into debate preparations than his opponents. He seemed to handily win that exchange with Cruz, a former college debate champion.

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BUSH STANDS ALONE

Pressed on whether regretted his call to bar Muslims from entering the U.S., Trump had a one-word answer: "No." And when asked whether they supported Trump's proposal, the candidates sharing the stage with him largely demurred, talking instead about immigration and security.

All expect Jeb Bush.

The former Florida governor has made taking on Trump the cornerstone of his struggling campaign, and he slammed the proposal as one that would make it impossible for the U.S. to build the necessary bridges with Arab nations to defeat the Islamic State.

"All Muslims? Seriously? What kind of signal does that send to the rest of the world?" Bush said. "What we need to do is destroy ISIS. The other Arab countries have a role to play in this. Sending that signal makes it impossible for us to be serious about taking out ISIS and restoring democracy in Syria."

Reminded that he once called Trump and his proposal unhinged, Bush replied: "Yeah, they are unhinged."

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ESTABLISHMENT LANE FENDER-BENDERS

With Trump and Cruz surging in many polls, the candidates seeking the support of more traditional Republican Party voters desperately hoped to break out.

None did. But not for lack of trying.

Bush tried to score points by going after Trump. Rubio went after Christie. Kasich called for unity. And Christie mixed it up with Cruz and Rubio.

The Christie-Rubio dynamic stood out at times, particularly when Rubio refused to condemn an ad from a super PAC supporting him that portrays Christie as similar to President Barack Obama. Rubio claimed that Christie "endorsed many of the ideas" that the president supports.

Christie strongly disagreed — and he said that while two years ago Rubio called him a "conservative reformer," now he is misrepresenting his record. "When you're a senator what you get to do is talk and talk and talk and no one can keep up to see if what you're saying is accurate or not," Christie said.

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SPEAKING OF CLINTON

The former secretary of state was mentioned often by all of the candidates, and the attacks were often in the form of one-liners.

"She's under investigation with the FBI right now," Bush said, adding that the country should avoid a situation with a president who "might be going back and forth between the White House and the courthouse."

Rubio was much more serious when he said Clinton is "disqualified from being president of the United States" because she "mishandled" intelligence, a reference to her use of a personal email account and server rather than a government account while at the State Department. He also accused her of telling "lies" to the families of victims in the attack on the U.S. outpost in Benghazi.

Separately, Christie warned, "If you're worried about world being on fire, you cannot give Hillary Clinton a third term of Barack Obama's leadership."

There was little love for Clinton's rival, Bernie Sanders. He may be surging in some preference polls, but the Republican candidates remained focused on facing off against Clinton this fall.

"We're going to win every state, if Bernie Sanders is the nominee," Kasich said, adding, "I know Bernie, and I can promise you he's not going to be president of the United States."

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