Here’s what to look out for during tonight’s debate.

But the two other candidates on stage, entrepreneur Vivek Ramaswamy and former New Jersey Governor Chris Christie, will do everything they can to grab the spotlight in hopes of reviving their flag campaigns.

Here’s what you can watch:

Who will stand out on a less crowded stage?

The “will he or won’t he” speculation over whether Mr Trump would participate in the previous debates in Wisconsin, California and Florida has advanced ahead of the Alabama meeting. The former president’s decision to rule out the events has not hurt his standing in the polls, and the question for many now is whether he will appear at a general election debate next fall.

But as the field shrinks due to attrition, the final four will have more time to make an impact on the yet-to-be-decided Republican primaries.

Gov. Doug Burgum of North Dakota on Monday became the latest candidate to withdraw from the race, despite failing to reach the stage for the final debate. South Carolina Sen. Tim Scott’s withdrawal will be even more keenly felt as he likely would have qualified for Wednesday’s event. His debate appearances have been largely unremarkable, but he made a small splash in Miami last month when he showed up with his girlfriend.

The most memorable lines of the last two debates involved Ms. Haley skewering Mr. Ramaswamy. In September she told her younger rival: “Every time I hear you I feel a little dumber,” and last month she called him “just scum.”

These lines raise an important question for Mr. DeSantis as he tries to fend off Ms. Haley’s rise in the polls: Can he take her more directly on and win?

It appears that Mr. Ramaswamy will continue his strategy of denigrating and challenging all his opponents except Mr. Trump, although the effectiveness of his insult-driven blitzkrieg appears to have waned since he shocked the field in Milwaukee in August. On Saturday in Iowa City, he said he had been “brutally outspoken in the last debate,” adding, “I have no intention of stopping now.”

Mr. Christie faces a loftier question: Is his stated goal of thwarting another Trump presidency better served by stepping down and letting a rival consolidate the anti-Trump vote?

Can Haley keep her winning streak alive?

The former South Carolina governor has parlayed her debate performances into a real sense of momentum. Yes, she remains far behind Trump, the man who made her his first U.N. ambassador, in the national polls, but her trajectory has been slow and steady, unlike that of her rivals on stage.

Wednesday’s debate is the first since the political network founded by billionaire conservatives Charles and David Koch endorsed Ms. Haley and vowed to mobilize an army of grassroots door-knockers behind her. It is also the first since Jamie Dimon, the CEO of JPMorgan Chase, began encouraging other major donors and Democrats to support her as a last hope to thwart Trump’s nomination.

She must reassure those new lenders that they have made a good bet. To do that, she will have to find the zingers she used to dismantle Mr. Ramaswamy and turn them against the candidate she now has in her sights, Mr. DeSantis.

However, she has yet to figure out whether she is the candidate for those within her party who fear and loathe Mr. Trump, or whether she wants to appeal to Trump supporters as a new face to take up his mantle. If she’s the first, she may only get so far in a Republican Party that still largely approves of the former president. Appealing to Trump likes and detesters has been the trick that no Republican has solved.

Can DeSantis take the microphone away from Ramaswamy?

After months of watching Trump, DeSantis reached out to him extensively on Tuesday.

He criticized Mr Trump for this bragging about the support of a Black Lives Matter activist, for criticizing Mr. DeSantis’ strong anti-abortion status, and for somehow blaming the governor of Florida for the College Football Playoff selection committee’s snub of Florida State University, which was not selected to compete for the championship despite an undefeated season. (The University of Alabama, in Tuscaloosa, benefited from that criticism, so look for college football talk on Wednesday nights.)

But Mr. DeSantis’ main criticism was Mr. Trump’s refusal to debate: “I don’t think he can stand there against me for two hours and come out the winner,” he said. “I think they know that, and I think that’s why they don’t do it.”

Clashing with Mr. Trump is critical; After all, you cannot win the nomination without beating the frontrunner. But Mr. DeSantis must also tone down Ms. Haley’s rise.

In Tuscaloosa, Mr. DeSantis has to take the microphone away from Mr. Ramaswamy, which has fallen to fourth place in national poll averages. Mrs. Haley, on the other hand, does firmly in second place in New Hampshire, neck and neck with Mr. DeSantis in Iowa and threaten him nationally.

Mr. DeSantis’ urgent task is to reaffirm his status as an alternative to Trump, and if he is to do that, the debate must not devolve into another cage fight between Ms. Haley and Mr. Ramaswamy.

What is Chris Christie’s endgame?

For reasons of ego, grim self-confidence or plans for his future, Mr. Ramaswamy, a political neophyte with barely an elected office to his name, is unlikely to leave the primary race anytime soon. The money he spends from his own bank accounts – $17 million as of September 30 – can keep his campaign going for as long as he wants.

Mr. Christie is in many ways the antithesis of Mr. Ramaswamy, a career civil servant without a vast fortune to tap, whose raison d’être for his campaign is to diminish Mr. Trump’s stature, not to glorify him as the greatest president of the 21st century. But the former New Jersey governor finds himself at a crossroads in Tuscaloosa.

He barely made it to the debate stage, but just barely qualified for the Republican National Committee’s tightening requirements — polling 6 percent or more in national or early state polls, and amassing 80,000 unique donors.

And his third-place status in New Hampshire, with about 12 percent of the vote, could be seen as a strength or a spoiler for the ambitions of the second-place candidate, Ms. Haley, who needs a strong showing in the elections. Granite State to launch her into the primaries in her home state of South Carolina.

Mr. Christie continues to denounce Trump’s fitness for office in ways his Republican rivals will not, challenging the former president as a would-be dictator who threatens to end democracy as we know it. But that line of attack has proven ineffective among Republican primaries.

Megyn Kelly is back. How will she deal with the absent Trump?

The 2016 presidential campaign may seem like ancient history, but for many Americans, Trump’s treatment of then-Fox News host Megyn Kelly during the debates cemented his reputation as a misogynist.

After Ms. Kelly questioned him vigorously during a debate, he came back with, “You could see there was blood coming out of her eyes, wherever blood was coming out of her.” Ms. Kelly was defiant toward angry Trump supporters, declaring that she “would not apologize for doing good journalism.”

Wednesday’s debate, which will be hosted by cable news newcomer NewsNation, will have a significantly smaller audience than those Fox showdowns in 2015 and no Mr. Trump — but Ms. Kelly, who now hosts “The Megyn Kelly Show” on Sirius XM , will be back.

She will undoubtedly be hard on the four participants. The question is: how hard will she push to take on Trump?

Ms. Kelly will share the moderators’ desk with Elizabeth Vargas of NewsNation and Eliana Johnson of the Washington Free Beacon, an all-female panel that is tilted to the right. The debate will be broadcast on television on the CW starting at 8 p.m. Eastern time, and streamed on the NewsNation website and conservative social media site Rumble.

Anjali Huynh And Maggie Astor reporting contributed.