As the debate turned to bans on gender-affirming medical treatments for minors, Christie offered a different view than the three other candidates on stage.

He said it should be a parent's decision, not the decision of the government.

"We should empower parents to be teaching values that they believe in their homes without government telling them what those values should be," he said. "Yet we want to take other parental rights away. I'm sorry, but as a father of four, I believe there is no one who loves my children more than me. There is no one who loves my children more than my wife. There is no one who cares more about their success in health, in life than we do, not some government bureaucrat."

Christie added: "This is not something I favor. I think it is a very, very dangerous thing to do. But that's my opinion as a parent. I get to make the decisions about my children, not anybody else."

Other candidates dove straight into their opposition to gender-affirming care for minors.

DeSantis said, "You do not have the right to abuse your kids." Ramaswamy said his view is that "transgenderism is a mental health disorder," which fact-checkers have challenged.

-ABC News' Alexandra Hutzler

“Father time is undefeated,” DeSantis told the debate audience twice, raising doubts over Trump’s age, at 77 years old, amid intervening attacks from Christie that he avoided the original question.

“He’s afraid to answer … this is the problem with my three colleagues,” Christie said. “They’re afraid to offend.” DeSantis wouldn’t respond to repeated questions by Christie and the moderators on the former president’s fitness for office.

“I’ll concede you’re fit, Ron,” Christie said. “You’re a new generation.”

- ABC News’ Chris Boccia

Addressing Trump's comments in an interview on Tuesday referring to himself as a "dictator" but only on "Day 1," Christie called the former president an "angry, bitter man who now wants to be back as president because he wants to exact retribution on anyone who has disagreed with him."

"Do I think he was kidding when he said was a dictator? All you have to do is look at the history," he said. "That's why failing to speak out against him, making excuses for him, pretending that somehow he's a victim empowers him."

Christie -- who Trump has dismissed as a failure both as a governor and candidate -- charged that the other hopefuls on stage "make it seem like his conduct is acceptable" because they said they would still support him if he was convicted of federal felonies, which Trump denies.

"Let me make it clear: His conduct is unacceptable," Christie said. "He's unfit."

Christie warned to "be careful of what you'll get."

"He will only be his own retribution. He doesn't care for the American people. It is Donald Trump first," he said, eliciting some boos from the crowd.

-ABC News' Meredith Deliso

Banking experts told PolitiFact that DeSantis’ claim about President Joe Biden pushing a “central bank digital currency” was dubious. Even if the system were technically feasible, current U.S. laws would not permit the kinds of monetary surveillance and control that DeSantis described, they told PolitiFact in April. The Federal Reserve is studying the possibility of creating a digital currency. But DeSantis’ remarks overstate the likelihood that such a system is possible, much less likely, to emerge in the United States -- for a variety of technical, legal and political reasons, experts told us.

-Analysis by Katie Sanders of PolitiFact

At the beginning of the year, DeSantis was the clear runner-up in the Republican presidential primary field: He was averaging above 30% in national polls, only about 10 points behind Trump. But DeSantis’ star has fallen the more voters have gotten to know him, it seems, and another candidate is now on the verge of overtaking him as the most viable Trump alternative: Haley.

As of Tuesday, Haley was at 11% in 538’s national polling average of the Republican primary -- just 2 points behind DeSantis for second place. And she’s doing even better in early primary states. She’s polling at 15% in Iowa, and she’s comfortably in second place in New Hampshire (19%) and South Carolina (21%).

She also recently secured the endorsement of Americans for Prosperity Action, a conservative group affiliated with Republican megadonor Charles Koch that is known for its sophisticated political operation.

Why has Haley been on the upswing? The simplest explanation is these debates. On the day of the first debate, she was averaging just 3% in national polls. But according to a 538/Washington Post/Ipsos poll conducted both before and after that debate, Republicans who watched said Haley was one of the strongest performers, and the share who said they were considering voting for her shot up by 17 points.

Republican debate watchers also rated Haley as the second-best performer in the second debate and the strongest performer in the third debate. Probably not by coincidence, her polling numbers have been steadily increasing ever since.

Of course, Haley fans shouldn’t get too giddy. She (and DeSantis) are still far behind Trump both nationally and in the early primary states. Even if Haley does overtake DeSantis and winds up being the last non-Trump Republican standing in the primary, it won’t matter if Trump is winning every state by double digits.

-Analysis by Nathaniel Rakich of 538