Here are five things to watch in the debate tonight.

The changing landscape will most likely alter the strategic calculations of the candidates who qualified under the Republican National Committee rules: Nikki Haley, the former South Carolina governor and former ambassador to the United Nations; Ron DeSantis, the Florida governor; Chris Christie, the former New Jersey governor; Vivek Ramaswamy, the entrepreneur; and Tim Scott, the South Carolina senator.

Here are some things to watch over the course of the two-hour debate airing on NBC.

It’s Nikki Haley’s Moment

Ms. Haley has drawn more attention in recent weeks, as other candidates — most notably former Vice President Mike Pence — have lost support or dropped out. She has a chance to use her momentum to eclipse Mr. DeSantis, her most serious rival in the race, to be the top Trump alternative.

“Haley is the now only candidate with a clear path to break through against Trump,” said Mike Murphy, a longtime adviser to Republican presidential candidates who is not involved in this campaign. He said he expected the debate to be “about her doubling down on her moment as the race is heating up”

Ms. Haley has a choice here: Will she devote more time to her attacks on Mr. Trump or to challenging Mr. DeSantis? Her campaign released a video on Tuesday attacking the governor on energy policy, which suggests that Mr. DeSantis should not expect an easy night.

Can DeSantis Still Play the Front-Runner?

At the first two debates, Mr. DeSantis played the front-runner, attacking his opponents only when he was hit first. That might not work anymore as he is under increasing pressure to slow Ms. Haley’s rise in the polls and reassure voters who may have come to question his political agility and strength as a general election candidate.

This has not been an easy stretch for Mr. DeSantis, in no small part because of the attacks from Mr. Trump on everything from his foreign policy credentials to his height. But Mr. DeSantis is on friendly turf in Miami: He won re-election as governor last year in a rout. And this week he drew the endorsement of Kim Reynolds, the governor of Iowa. Mr. DeSantis has staked his bid on his performance in the state’s first-in-the-nation caucuses on Jan. 15.

But in a sign that Mr. DeSantis’s status may be diminishing, Mr. Christie said he was likely to largely ignore his rival tonight.

“What do you attack?” he said in an interview. “If he says something which I think is worthy of being responded to, I’ll respond to it. But I’ve now spent four hours on the debate stage with him, and I haven’t heard him say one thing worthy of being responded to.”

A World in Turmoil

Foreign policy, with some noteworthy exceptions over the years, has not proved determinative in presidential nominating contests. But the war in Ukraine and the bloodshed in the Middle East are likely to feature prominently at the debate on Wednesday.

The question of U.S. assistance to Ukraine has divided the Republican Party, and could display clear differences among the candidates over whether they would follow Mr. Trump’s isolationist, populist path. The candidates are likely to be pressed on whether they back House Speaker Mike Johnson’s first major proposal — a plan to tie money for Ukraine to a border bill unpopular with Democrats.

While the Republican Party is more unified in its support for Israel (in contrast with the Democratic Party), the conflict has prompted some of the field’s sharpest criticism of Mr. Trump.

At the Republican Jewish Coalition gathering last month, Ms. Haley, who has more experience in foreign policy than her rivals on the stage, attacked Mr. Trump for calling Hezbollah “very smart” and describing Prime Minister Benjamin Netanyahu of Israel as weak days after the deadly attack by Hamas on Israeli settlers.

Mr. DeSantis offered a similar criticism of the president while campaigning in New Hampshire in October. “Now is not the time to be doing like what Donald Trump did by attacking Prime Minister Bibi Netanyahu, attacking Israel’s defense minister, saying somehow that Hezbollah were ‘very smart,’” he said.

Does Anyone Else Have a Shot?

Of the three undercards, Mr. Scott is, in the view of Republicans, the only one who seems to have much chance of breaking through. Until now he has been blotted out by higher-profile opponents, and the likelihood that this debate will be focused on Ms. Haley and Mr. DeSantis will not make things easier for him.

Mr. Ramaswamy proved to be an energetic debater in the first debate, but by the second debate he became more of a target. As he registers in the single-digits in many polls, he does not loom as a major force in the race going into tonight. And Mr. Christie could hardly be more out of step with much of the Republican Party with his relentless attacks on Mr. Trump: He is routinely booed at Republican events.

Is Anybody Out There?

One major question is how many people will even be watching. Viewership dropped to just under 10 million in the second debate from 12 million in the first debate. Unless Mr. Trump makes a dramatic last-minute appearance on the stage, that seems unlikely to change.

The waning audience is perhaps not a surprise given Mr. Trump’s dominance. With Mr. Trump enjoying wide leads over the rest of the Republican field in most polls, the race can feel like it’s over before a single vote is cast — even though large swaths of Republicans have said they are at least open to nominating someone other than Mr. Trump.