He won't be there. But Trump is expected to haunt tonight's Republican presidential debate

He won't be therephysically,but Donald Trump willcertainly loom large over the first Republican presidential candidate debate tonight in Milwaukee. That means, despite the absence of the former president, theother candidates on stage could face some significant challenges trying to capture just some of the spotlight often reserved for Trump.

"It's a challenge because whether it's on the debate stage or on the campaign trail, President Trump is taking up a lot of oxygen," said Matt Terrill, former chief of staff for Marco Rubio's campaign for president.

Even if he's not on stage, Trump "still will be taking up a lot of oxygen."

Eight other candidates met the donor and polling qualifications to be on stage, according to the Republican National Committee. They are:

• Florida Gov. Ron DeSantis.
• Former vice-president Mike Pence.
• Entrepreneur Vivek Ramaswamy.
• South Carolina Sen. Tim Scott.
• Former South Carolina governor and UN ambassador Nikki Haley.
• Former New Jersey governor Chris Christie.
• Former Arkansas governor Asa Hutchinson.
• North Dakota Gov. Doug Burgum.

The former president, who, according to polls, enjoys a commanding lead over his rivals, has opted not to participate in the two-hour debate (or any other debate for that matter), likely seeing little to gain, and potentially more to lose with a bad performance.

"I don't think there's anyway you can overcome his presence even if he is physically not on the stage," said Alan Schroeder, professor emeritus at Northeastern University's school of journalism, and an expert on presidential debates. "I think he'll dominate."

WATCH | Why Trump is skipping the debate:

Why Trump is skipping the Fox News Republican presidential debate

1 year ago

Duration 7:16

Even though Donald Trump is opting out of the first GOP debate, his presence will still loom large. Andrew Chang explains why Trump would have little to gain and lots to lose by taking the debate stage.

The debate moderators, Fox News anchors Bret Baier and Martha MacCallum, have already indicated they will lead with Trump as their topic, meaning "there's not much getting around him," Schroeder said.

Trump's absence will also rob his opponents of opportunities to score points against him.

Some, including DeSantis and Ramaswamy have recently shown more willingness to criticize the former president.

"Frankly, I think he might be scared of being on a debate stage," Ramaswamy recently told ABC News.

Still, it's unlikely Trump would have faced heavy criticism, or been put on the defensive. Trump enjoys such popularity among a core group of Republican primary voters that most of the presidential candidates, in general, have been reluctant and fearful to attack him on the campaign trail, worried about alienating potential support.

"His presence there would really ratify the a long and sustained support for him by everyone in the Republican Party, including those other candidates who are there," said Ed Lee III, senior director of the Alben W. Barkley Forum for Debate, Deliberation and Dialogue at Emory University.

"I think that his absence actually does them a favour, where they don't have to answer questions in support of him while he's on the stage."

A Trumpless debate could also have a drastic impact on television ratings, meaning a much more limited potential to gain name recognition for the other eight. Still, the two hours without Trump "really opens up time and attention the other candidates wouldn't have got if he was there," said Aaron Kall, the Lee H. Hess Director of Debate at the University of Michigan

"If others can emerge from Trump's shadow and they get some momentum for their campaigns, maybe rise in the polling a little bit ... it may kind of dare him to come back in the ring."

Another DeSantis reset?

The debate gives Florida's governor, whose support has plummeted from just a year ago, another chance to redefine the trajectory of the race.

But because he's running in second place, far behind Trump, being on stage without him could also put a target on his back.

He's not known as a particularly gifted debater and was criticized in his debates against Charlie Crist for the governorship of Florida for looking bored, like he was just going through the motions.

That means, Krall said, hisphysical presentation will be very important. Schroeder says his advice to DeSantis would be to stay on the offence.

"Try not to be put in that defensive posture where you're just having to deflect, you know, incoming artillery from your opponents, and try to get your message out positively."

DeSantis has indicated he is taking the debates seriously, having hired veteran presidential debate coach Brett O'Donnell to help

But just a week before tonight's contest, memos about DeSantis'sdebate strategy were posted online by a super PAC supporting him. As reported by The New York Times, the advice included taking a "sledgehammer" toRamaswamy, attacking U.S. President Joe Biden and the media at leastthree to five times, and to"defend Donald Trump" when Christie inevitably attacks the former president.

Butkeeping the attacks away from Trump and punching down onRamaswamywould beill advised, Schroeder says.

"You don't want to give attention to somebody who's running behind you, and you better criticize the person who's running ahead of you if you expect to take away any of their voters."

Time for Vivekto shine?

Polls indicate thatRamaswamyis gaining some momentumthe candidate with no political experience much like the current front runner when he first entered the political fray.

This could be a good opportunity for him to really boost his name recognition, and build upon that momentum

"He's kind of the wild card here because, of course, he's never run for office.So nobody much knows what to expect of him," Schroeder said.

Not only does he lack political experiencebut, at age 38, is quite a bit younger than the other candidates, and half the age of the former president.

"So I think he'll probably lean pretty heavily on his biography. There's not a lot of daylighton his policy views between him and Trump, sort of accepts the Trumpian vision of the world," Schroeder said.

"I think he's intriguing to a lot of people because he's a non-politician. And so in a way, he has the same opportunity that Trump had in 2016 as the sort of truth teller or the the guy who does not play the typical political games."

Butmost importantly, Ramaswamyhas to look presidential, Lee said.

"He wants to look the part of being someone who can govern, someone who can manage, and someone who has an understanding of the way the roles of the federal government."

Added Kall: "He's got to make sure that he can make people feel comfortablethat he could both go toe to toe with President Biden on a big stage and andfeel comfortable with him at the helm."

Without Trump, what's Christie to do?

Of all the candidates, the former New Jersey governor has been the most criticaland most vocal with his criticism of Trump.

Christie, a former prosecutor, is also considered a skilled debater who helped derail Rubio's campaign in 2016 because of this repeated jabs at the Florida senator.

"He's very knowledgeable. He has executive governance chops. ButI don't think that anyone else sees him as a viable candidate," Lee said. "He has the ability to be the person who probably loses the most by Donald Trump not being at these debates."

Kall says Christie will need to not spend too much time worrying about Trump, given he's not there.

"How much credit do you get for attacking someone that can't defend themselves?" Kall said. "Especially if you have other people kind of rising to his defence?"

"He's got to find a favouritenew target that actually is there in person."