DeSantis expressed support for using the U.S. military to combat drug cartels across the border.

"The president of the United States has to use all available powers as commander in chief to protect our country and the people," DeSantis said.

"When they are coming across, yes we are going to use lethal force," he continued -- a line he's used repeatedly throughout the campaign.

Activists have pushed back that his plan is inhumane and illegal.

-ABC News' Alexandra Hutzler

Martha MacCallum just asked about what to do about migrants already in the United States. In a July poll by The New York Times/Siena College, 39 percent of likely Republican voters say that they supported a “comprehensive immigration reform bill that provides a pathway to citizenship for all undocumented immigrants” in the United States, while 57 percent opposed such a bill. Support was weakest among Trump supporters (29 percent) compared with DeSantis supporters (48 percent) and voters who say they will vote for another candidate (50 percent).
-Analysis by FiveThirtyEight

While many candidates onstage have sparred with each other -- including Haley and Ramaswamy swapping sharp words on foreign policy -- Scott has yet to engage with any of the other candidates.

Much of his remarks have almost word for word repeated elements from his campaign stump speech. When asked about China, which he has addressed in other appearances, he pivoted to speaking about the U.S. border.

-ABC News’ Gabriella Abdul-Hakim and Oren Oppenheim

Asked to raise their hand if they would not increase aid to Ukraine, DeSantis did so, saying that "Europe needs to step up. They need to step up and do their job."

He argued: "I would have Europe pull their weight. ... Our support should be contingent on them doing it.

DeSantis -- who had received backlash from some other politicians for seemingly backtracking on support for Ukraine -- pivoted to say that he wasn’t going to send troops overseas but rather down to the Southern border.

-ABC News' Hannah Demissie and Will McDuffie

DeSantis remains Trump’s closest challenger, but his support has been dropping over the past several months: He currently trails the former president by nearly 40 percentage points in our national polling average.

With that drop have come some shake-ups in the DeSantis campaign. The campaign has laid off around one-third of its 92-person staff, with the latest cuts coming in July, according to ABC News. Changes stretched all the way to the top: DeSantis replaced his campaign manager with his governor’s office chief of staff. On top of that, some of his top donors have voiced concerns, saying they want DeSantis to recruit more major donors and take a more moderate approach on social issues.

All of that has made this debate crucial for DeSantis, both in terms of trying to chip away at Trump’s lead and proving to his supporters that he actually has a shot. He’s recruited a veteran debate coach and is getting advice from a supportive super PAC, according to The New York Times. That advice? Rather than attack the front-runner, DeSantis should “take a sledgehammer” to Ramaswamy who, unlike DeSantis, is rising in the polls. DeSantis won’t just be trying to win the debate tonight: He’s also aiming to push back against the rolling tide of news that hints at a flailing campaign.

-Analysis by Monica Potts of FiveThirtyEight