From businessman Donald Trump's slam on Rosie ODonnell to Gov. Chris Christie (R-N.J.) and Sen. Rand Paul (R-Ky.) getting into it over hugs, here are some of the most memorable moments from first Republican presidential debate. (Fox News Channel)

Businessman Donald Trump lived up to his sharp-edged reputation during the first Republican debate of the 2016 presidential campaign, sparring with moderators and other candidates as everyone else on a 10-person stage struggled to stand out.

Trump became the center of the debate’s attention from the very beginning, when he was the only candidate who refused to forswear the idea of running a third-party campaign against the Republican party, if he could not be its nominee.

“I cannot say, ‘I have to respect the person, who is not me,’” Trump said, as the crowd booed him. “We want to win, and we will win. But I want to win as the Republican. I want to run as the Republican nominee.”

Immediately, Sen. Rand Paul (Ky.) attacked, saying that Trump was “hedging his bets,” and accusing him of being too close to the Clinton family. “He’s already hedging his bets on the Clintons,” Paul said, pointing in Trump’s direction. “He’s already hedging his bets, because he’s used to buying politicians.”

As the debate proceeded, it seemed to veer between two broad topics: America, and Donald Trump.

At times, the other nine candidates on stage debated serious policies—immigration, the nuclear deal with Iran, government surveillance, the future of Social Security. And then, at times, the debate veered to Trump himself: a sharp-edged candidate who can say things that would torpedo anybody else on stage.

Earlier in the debate, Trump defended himself against questions about his companies’ multiple bankruptcy filings during Thursday night’s first major Republican primary debate, saying that he had “taken advantage” of the country’s bankruptcy in order to help his businesses.

“Out of hundreds of deals—hundreds--on four occasions, I’ve taken advantage of the laws of this country, like other people,” Trump said, in response to a question from moderator Chris Wallace. “The difference is, when somebody else uses those laws, nobody writes about it. When I use it, it’s like, ‘Oh, Trump, Trump, Trump.”

In one of the non-Trump-related exchanges, New Jersey Gov. Chris Christie and Paul (got into a heated argument about the limits of government surveillance during Thursday night’s first major Republican debates, in an exchange that showcased two competing poles of Republican thought about security and privacy.

The GOP debate, charted word by word

“I want to collect more records from terrorists, but less records from innocent Americans,” said Paul, the son of libertarian icon Ron Paul, and one of the party’s strongest advocates for limiting government collection of Americans’ phone records and other data.

“That’s a completely ridiculous answer,” said Christie, a former federal prosecutor, who has said that the Sept. 11, 2001 terrorist attacks illustrated the need for broad-ranging collection of intelligence. “How are you supposed to know?”

“Use the Fourth Amendment! Get a warrant!” Paul responded.

Christie countered with the age-old insult that governors use against senators: “When you’re sitting in a subcommittee, blowing hot air about this,” the problem might seem easy, he said.

Paul retorted with the age-old insult that other Republicans use against Christie. “I don’t trust President Obama,” with records, he said. “I know you gave him a big hug.”

On a stage with 10 candidates, some seemed almost to disappear—neurosurgeon Ben Carson, Wisconsin Gov. Scott Walker, and Sen. Ted Cruz (Texas) all struggled for air time. Others broke with the hard-edged cultural conservatism that dominated the 2012 Republican primary. Ohio Gov. John Kasich said he had attended a same-sex wedding, and described that the Supreme Court decision legalizing same-sex marriage should be accepted as the law of the land.

Former Florida governor Jeb Bush defended an earlier statement calling immigration—including illegal immigration—“an act of love.”

“I believe that the great majority of people coming here illegally have no other option, and they want to provide for our family. But we need to control our border,” Bush said, pivoting to his plans to crack down on illegal immigration. He also returned to an idea that other Republicans have rejected: “There should be a path to earned legal status,” for illegal immigrants already in this country, Bush said. Others have rejected any pathway to legal status as “amnesty.”

But, time and again, the debate returned to Trump—and his long history of over-the-top statements, and flirtations with Democrats and Democratic ideas. At one point, Trump reiterated what—for any other candidate—would be a radioactive statement. He liked nationalized-single-payer health-care system—at least, as it worked in other countries.

“It works in Canada. It works incredibly well in Scotland. It could have worked” in the U.S. at one point, Trump said. Still, he said, he now supports a more modest set of health-care reforms, including allowing consumers to buy insurance across state lines.

Paul spoke up, saying that Trump was on the wrong side of everybody by praising a single-payer system. Trump brushed him off. “I don’t think you heard me. You’re having a hard time tonight,” he said.

In the debate’s second hour, there was a civil exchange between Christie and former Arkansas governor Mike Huckabee about how to reform Social Security. Christie urged some reforms: raising the age at which seniors are eligible for the benefits, and allowing well-off seniors to collect less. Huckabee resisted those changes, saying that any reduction in anyone’s Social Security benefits was “fundamentally lying to the people, and stealing from them.”

“He’s complaining about the lying and stealing--the lying and stealing has already occurred,” Christie said, meaning that the Social Security trust fund was already under-funded.

Earlier in the debate, Trump took credit for bringing the subject of immigration into the 2016 presidential campaign during the first major GOP candidates’ debate on Thursday evening, in a sharp-edged performance in which he also indicated he might run as a third-party candidate if Republicans don’t choose him.

“If it weren’t for me, you wouldn’t even be talking about illegal immigration, Chris. You wouldn’t be talking. This wasn’t a subject,” Trump said to one moderator, Fox News’ Chris Wallace. Wallace also tried to press Trump to produce evidence for a key Trump claim: that the Mexican government was actively dispatching illegal immigrants over the border. Trump cited only conversations with “Border patrol. People, that I deal with, that I talk to, they say, this is what’s happening.”

He said he remained convinced that the Mexican government was orchestrating immigration, in order to avoid paying benefits and other costs associated with its own citizens. “The stupid leaders of the United States will do it for ‘em, and that’s happening, whether you like it or not.”

At the beginning of the debate, Paul had shown himself willing to attack Trump. But not everyone thought that was wise.

“They say we’re outspoken, we need to take lessons from Donald Trump,” Ohio Gov. John Kasich said, when Wallace asked him to critique Trump’s assertion. “He’s hitting a nerve.”

Sen. Marco Rubio (Fla.), also prompted to criticize Trump, also refused.

“People are frustrated,” he said.

As the debate’s first question, moderators asked candidates to raise their hand if they would not forswear running a third-party campaign against the GOP candidate. None of the 10 onstage raised their hand. And then, after theatrical pause, Trump did.

“I cannot say, ‘I have to respect the person, who is not me,’” Trump said, as the crowd booed him. “We want to win, and we will win. But I want to win as the Republican. I want to run as the Republican nominee.”

Immediately, Paul attacked, saying that Trump was “hedging his bets,” and accusing him of being too close to the Clinton family. “He’s already hedging his bets on the Clintons,” Paul said, pointing in Trump’s direction. “He’s already hedging his bets, because he’s used to buying politicians.”

Just as in 2012, the primary showcased the GOP’s combative side. The crowd cheered when a moderator mentioned that Cruz had called Senate Majority Leader Mitch McConnell (Ky.)—one of the top Republicans in Washington—a “liar.” And onstage, Trump continued to be the best at embodying that edge. When moderator Megyn Kelly asked Trump about past statements criticizing women for their appearance, Trump responded by saying, “Only Rosie O’Donnell.”

He then turned on Kelly herself, suggesting she was on thin ice by even asking the question.

“I don’t frankly have time for total political correctness. And to be honest with you, this country doesn’t have time, either,” Trump said, as the crowd cheered. “If you don’t like it, Megyn, I’m sorry. I’ve been very nice to you. Although I could maybe not be, based on the way you have treated me.”

Other candidates sought to distinguish themselves in the debate’s first few minutes. Huckabee attacked Planned Parenthood by saying that it sold parts of aborted fetuses “like parts to a Buick.” Rubio, whose parents were Cuban immigrants, said that he could debate Hillary Rodham Clinton about what’s best for families living paycheck-to-paycheck, because he had lived that way.Bush—the son and brother of presidents—responded to a question about his family legacy by saying that “They called me Veto Corleone,” in Florida, he said, because he had vetoed so many bills. “I’m my own man.”

The two-hour debate, which began at 9 p.m. on Fox News Channel, ties the record for most candidates in any primary-season debate. And that’s not even the whole field: earlier Thursday evening, seven other, lower-polling candidates held a separate debate in the same Cleveland arena.

In that undercard, former tech executive Carly Fiorina stood out, with pointed attacks on Democratic front-runner Hillary Rodham Clinton--and on Trump himself.

“I didn’t get a call from Bill Clinton before the campaign,” she said, referring to a phone call reported by the Washington Post on Wednesday, in which Clinton encouraged Trump to get more involved in GOP politics. As it turned out, Trump has kneecapped the major projected rivals for the Democratic front-runner, who happens to be Clinton’s wife.

Fiorina also noted Trump’s history of position changes: “Since he has changed his mind on amnesty, on health care and on abortion, I would just ask, ‘What are the principles on which he will govern?'”

In the evening’s first debate among second-tier candidates, the person who seemed to do herself the most good was Fiorina, the only woman in the debate, and the only non-politician on a stage full of current and former senators and governors.

Fiorina talked about her experience meeting with foreign leaders, and urged greater cooperation with Israel, and with Arab countries that want to fight the Islamic State. She said she would personally call Iran’s Supreme Leader on her first day in office, to let him know that the U.S. would insist on tougher inspections of nuclear facilities, regardless of the deal recently negotiated by the Obama administration.

Fiorina said that, under her leadership, people would know “America is back in the leadership business.”

She stood out.

But, in this undercard, it was a low bar. The candidates largely agreed with each other, about wanting to undo President Obama’s policies on health care, immigration and Iran. The next 10 candidates will likely say something similar.

And, in this debate, the seven candidates had to begin by answering some version of a humiliating question: If you’re here, at the matinee, why don’t you just give it up?

“Has your moment passed, senator?” a moderator asked former Pennsylvania senator Rick Santorum at the beginning.

“If the people of Louisiana are not satisfied, what makes you think the people of this nation should be?” a moderator asked Gov. Bobby Jindal (R-La.), after describing his dismal poll numbers.

“Is it time for new blood?” was the question for former Virginia governor Jim Gilmore, who hasn’t run for office in more than a decade.

Did you miss your moment? the moderator asked former New York governor George E. Pataki, who had considered, then abandoned a presidential bid in the past. Pataki’s answer was a strange contortion: “I was ready to lead” back then, Pataki said. “But I wasn’t ready to run.”

The most pointed moment came when Santorum compared the recent Supreme Court decision legalizing same-sex marriage to one of the most infamous decisions in American history.

Asked if the same-sex marriage decision was “settled law,” Santorum responded, “It is not, any more than Dred Scott was settled law to Abraham Lincoln.”

He meant the 1857 Supreme Court decision in Dred Scott vs. Sandford, in which the court declared that African Americans could not be citizens. President Lincoln later--during the middle of a Civil War fought over the issue of slavery--issued the Emancipation Proclamation, freeing slaves in defiance of the Scott ruling’s racist logic.

For the most part, however, the group did little to move themselves out of the back of the pack.

Meanwhile, Democrats announced Thursday that their debate season will kick off Oct. 13 in Nevada. The first debate will be followed by five others for the Democrats, a schedule that led a pair of underdog candidates to swiftly complain that there should be more.

Ed O’Keefe and Philip Bump contributed to this report.