MILWAUKEE -- He'll be missing from the stage, but Donald Trump will be a central figure at the first Republican presidential primary debate Wednesday night as the remaining candidates hoping to take on President Joe Biden confront each other in person for the first time.

The eight contenders meeting in Milwaukee for the event hosted by Fox News will likely face pressure to articulate how they would differ in style and substance from Trump, who holds a commanding early lead in the race. That could be a delicate task, forcing candidates to decide how closely to align themselves with the former president's most outlandish positions, including his lies about widespread fraud during the 2020 election.

With less than five months until the Iowa caucuses jumpstart the GOP presidential nomination process, the debate is a critical opportunity for lower-polling candidates to introduce themselves to millions of voters, many of whom are just beginning to pay attention to the race. The pressure is perhaps greatest for Florida Gov. Ron DeSantis, who announced his campaign in May to great fanfare but has since struggled to gain traction and is now fighting to maintain his distant second-place status.

Beyond DeSantis, the debate will include South Carolina Sen. Tim Scott, former U.N. Ambassador Nikki Haley, biotech entrepreneur Vivek Ramaswamy, former New Jersey Gov. Chris Christie, former Vice President Mike Pence and former Arkansas Gov. Asa Hutchinson. North Dakota Gov. Doug Burgum hurt his Achilles tendon playing basketball with members of his campaign staff Tuesday and was taken to the emergency room, but said he will participate nonetheless.

The primetime event was unfolding at a moment of reckoning for the Republican Party.

Trump is now the prohibitive early front-runner in the race, raising serious questions about whether the party will have much of a competitive primary. Yet Trump's vulnerabilities in a general election are clear, particularly after four criminal indictments that charge him with hoarding classified documents, conspiring to overturn the 2020 election and making hush money payments to a porn actor and other women.

The debate was taking place a day before Trump is set to travel to Georgia to be booked again on criminal charges.

Yet Trump's standing in the primary has only increased as the charges have mounted, leaving the GOP on track -- barring a stunning realignment -- to nominate a candidate who would enter the race against Biden, a Democrat, in a potentially weak position. Polling this month from The Associated Press-NORC Center for Public Affairs Research found 64 per cent of Americans are unlikely to support Trump if he is the GOP nominee, including 53 per cent who say they would definitely not support him and 11 per cent who say they would probably not support him in November 2024.

The debate was being held at the Fiserv Forum in downtown Milwaukee, the arena that is home to the Milwaukee Bucks basketball team. The city will also be the site of next summer's Republican convention, a sign of the state's premier battleground status.

The Republican National Committee had set polling and donor thresholds and required participants to sign a loyalty pledge in order to qualify. Fox's Bret Baier and Martha MacCallum will moderate.

Trump had long said he felt it would be foolish to participate, given his dominant lead in the race.

His decision to boycott was nonetheless a blow to the network, which had wooed him privately and publicly to appear. Instead, Trump pre-recorded an interview with ex-Fox host Tucker Carlson that will be posted to the platform formerly known as Twitter as counter-programming.

Even in Trump's absence, his presence was expected to be felt at the debate. MacCallum had made clear she would press his rivals to respond to Trump's indictments, telling Vanity Fair that, "It will absolutely be incumbent upon them to address" them.

In a statement, Trump senior campaign adviser Chris LaCivita declared that Trump had "already won this evening's debate because everything is going to be about him."

"In fact, tonight's Republican undercard event really shouldn't even be called a debate, but rather an audition to be a part of President Trump's team in his second term," he quipped.

So far, the candidates have been reticent to take Trump on directly, given his broad support from the GOP base, and one key question will be how aggressively his rivals take him on.

In his absence, DeSantis' campaign had prepared for the Florida governor to be the debate's top target as the front-runner on stage.

In a memo sent to donors and supporters over the weekend, DeSantis campaign manager James Uthmeier said they were "fully prepared for Governor DeSantis to be the center of attacks," casting the contest as "a two-man race" for the nomination.

Uthmeier said DeSantis would try to remain above the fray, and that his "objective in this debate will be to lay out his vision to beat Joe Biden, reverse American decline, and revive the American Dream."

Also expected to be a target for criticism is Ramaswamy, who has been gaining ground in some polls. Never Back Down, the Super PAC supporting DeSantis, had advised the candidate to defend Trump and instead "Take a sledge-hammer" to Ramaswamy, who recently suggested the federal government might have been involved in the Sept. 11 terrorist attacks.

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Colvin reported from Washington. Associated Press writers Bill Barrow in Atlanta and Steve Peoples and Michelle Price in New York contributed to this report.