The moderators asked a question about Trump’s 2017 executive order barring immigrants from seven predominantly Muslim countries. That order led to nationwide protests and was highly divisive: 53% of Americans in a CNN/ORC poll said they opposed it at the time, while 47% said they supported it.

-Analysis by Nathaniel Rakich of 538

Crossing into the second hour of the debate, attention turned to Trump as Haley was asked if she agreed with his plan to bring back and expand his restrictions on immigration from Muslim countries.

Haley said there shouldn't be a straight ban on immigrants from Muslim countries, but rather a review of countries that have terrorist activity and represent a threat to the U.S.

"You look at what came across the southern border, what worries me the most are those that came from Iran, from Yemen, from Lebanon, those areas where they say 'death to America,'" she said. "That's where you want to be careful. It's not about a religion. It's about a fact that certain countries are dangerous and are threats to us."

DeSantis then stepped in, saying he would take it further.

"It's not just terrorism though," DeSantis said, adding: "There needs to be limits on immigration, and we should not be importing people from cultures that are hostile."

-ABC News' Alexandra Hutzler

The candidates were asked how tough they will be on high immigration at the southern border -- including the issues of unauthorized crossings and fentanyl being smuggled in.

DeSantis defended comments that he would order the military to shoot suspected drug smugglers "stone cold dead," despite critics saying that would violate the law, and he said he supports building a wall across the southern border

"The commander in chief not only has a right, you have a responsibility to fight back against these people," he said.

Haley was asked to clarify her stance on whether she would deport all unauthorized migrants. She said the U.S. has to "stop the incentive of what's bringing them over here in the first place" -- noting that President Joe Biden recently gave temporary protective status to 500,000 Venezuelans -- and also vet those who are in the country, for instance, based on their tax and employment status.

On the fentanyl overdose crisis, she turned to China, saying, "We need to end all normal trade relations with China until they stop murdering Americans with fentanyl."

Ramaswamy was asked how effective his policy to use the military to "annihilate drug labs inside Mexico" would be.

"I don't think it’s going to have to come to that if we deal with the actual demand-side problem that we also have in this country," he said. "I mean, the easy part is talking about how we’re going to use our military to secure the border. I will, and I believe that everybody else wants to do the same thing. But the harder part is dealing with the crisis of purpose and meaning, the mental health epidemic raging across the country like wildfire."

He also pointed to China as a source of fentanyl, saying, "We also have to hold them accountable with every financial lever that we have available."

-ABC News' Meredith Deliso

Haley lamented the state of economy, promising to veto any spending bill that doesn’t take federal expenditures to pre-COVID-19 levels. She slammed the Trump administration for adding $9 trillion to the federal debt.

DeSantis wouldn’t directly target Trump but chalked the the problem up to “both parties in Washington, D.C. That’s just a fact.” He pointed to student loans as an economic backbreaker for young people and touted his record on vocational education, which he said could encourage universities to become more accessible: “Don’t let anybody tell you the only way you can be successful is through a four-year, brick-and-ivy degree. That’s one way you could be, not the only way, and we’re gonna fix that problem in the United States of America.”

- ABC News’ Chris Boccia

Can anyone take down Haley? According to 538’s polling with The Washington Post and Ipsos, Republican primary voters have responded favorably to each of Haley’s first three debate performances, and it’s probably not a coincidence that she’s been rising in the polls at the same time. But that could put a target on her back on Wednesday -- and it’s possible that her momentum will stall if she’s perceived to have a bad performance.

Will anyone go after Trump? So far, the debates have mostly consisted of the candidates on stage attacking each other. But that’s an odd strategy considering that their biggest obstacle to the nomination is Trump. In fact, he’s above 50% in 538’s national polling average -- meaning that even if another candidate consolidates all of the non-Trump vote, Trump would still be ahead. That means that someone is going to have to peel support away from Trump in order to win the nomination -- and one way to do that is to attack him in the debates.

Who will perform best on the economy and immigration? According to the latest 538/Washington Post/Ipsos poll, 52% of likely Republican voters said getting inflation or costs under control was one of the top three issues in determining their primary vote; 41% said controlling immigration was one of their top three issues. These will probably be the most important segments to watch at the debate then: If a candidate really aces a question on inflation or immigration, that could go a long way.

Will anyone watch? The ratings have been falling with every debate: While the August faceoff garnered 12.8 million viewers, the September one got 9.5 million and the November one got only 7.5 million. And only 26% of the respondents to our 538/Washington Post/Ipsos poll said they were very likely or absolutely certain to watch the fourth debate. Obviously, the fewer people who watch the debate, the less anything that happens on stage will matter.

-Analysis by Nathaniel Rakich of 538