Former Gov. Chris Christie of New Jersey suspended his presidential campaign on Wednesday, but undermined his effort to stop Donald J. Trump when he sweepingly dismissed his rivals during a hot-mic moment.

Minutes before his announcement in Windham, N.H., Mr. Christie’s voice could be heard on the event’s livestream, saying, “She’s going to get smoked, and you and I both know it,” in a reference to Nikki Haley, the former South Carolina governor. “She’s not up to this.” He added of Gov. Ron DeSantis of Florida, “DeSantis called me, petrified.”

Mr. Trump immediately seized on the remarks, writing on Truth Social that Mr. Christie’s comments about Ms. Haley, who appears to be the most significant obstacle to a Trump victory in New Hampshire, were a “very truthful statement.”

Mr. Christie did not endorse any of his rivals in his speech, nor did he address their prospects against Mr. Trump, dashing the hopes of Republican moderates that his exit would mark a moment of coming together for remaining members of the party who oppose Mr. Trump.

In fact, Mr. Christie denounced his rivals’ long-running public deference to the former president, and offered no positive remarks about their candidacies.

“I would rather lose by telling the truth than lie in order to win,” he said. “And I feel no differently today because this is a fight for the soul of our party and the soul of our country.”

His departure effectively ends a phase of the Republican presidential contest, removing from the field its most aggressive Trump opponent and the only prominent contender who had argued that Mr. Trump was unfit for office — a contention that all but doomed his candidacy from the start.

“Imagine just for a moment if 9/11 had happened with Donald Trump behind the desk,” Mr. Christie said on Wednesday. “The first thing he would have done was run to the bunker to protect himself. He would have put himself first, before this country.”

Mr. Christie also acknowledged regret for his actions after Mr. Trump defeated him in the 2016 primary race. Soon afterward, Mr. Christie shocked the political establishment by endorsing Mr. Trump, becoming the first significant former candidate to back him as his march to the nomination picked up pace.

“For all the people who have been in this race, who have put their own personal ambition ahead of what’s right, they will ultimately have to answer the same questions that I had to answer after my decision in 2016,” Mr. Christie said. “Those questions don’t ever leave. In fact, they’re really stubborn. They stay.”

This is a developing story; check back for updates.

After months of relentless taunting and hyping a debate clash with former President Donald J. Trump, former Gov. Chris Christie of New Jersey turned to chiding.

“If you qualify for the stage, which Trump has, not showing up is completely disrespectful to the Republican Party, who made you their nominee twice, and to the Republican voters, whose support you’re asking for again,” Mr. Christie told reporters on Friday outside the famed Versailles Cuban restaurant in Miami.

Mr. Christie built his entire presidential candidacy toward a marquee confrontation with Mr. Trump, relentlessly goading him and needling him as a coward in a clear effort to tempt the quick-to-anger former president into showing up to the debate on Wednesday in Milwaukee.

It appears Mr. Trump will not take the bait, other than swiping back at Mr. Christie on social media and in speeches. Last week, he signaled that he planned to skip the first Republican debate and instead sit for an interview with Tucker Carlson that will be broadcast online at the same time.

Mr. Trump’s absence could lead to an anticlimactic scene at the debate, with Mr. Christie forced to launch unrequited broadsides through the airwaves without the fireworks of a Trump response.

“They have been taunting each other back and forth on Twitter and campaign town halls, so it robs Christie of a big moment that he is looking for if Trump doesn’t show up onstage,” said Ryan Williams, a Republican strategist and veteran of two presidential campaigns. “You can use the debate to get the anti-Trump message out that he’s pushing, but you’re going to lack that viral moment if the two of them aren’t looking at each other face to face.”

Yet Mr. Christie and his team also see an opportunity if the pugnacious and unpredictable former president is not onstage. Mr. Christie, a confident debater and the only Trump critic in the Republican field with any kind of foothold, could shine in the vacuum, using candidates who have been far more deferential to Mr. Trump as a stand-in for him.

Mr. Christie tried out that kind of approach at a town hall event in Miami on Friday, chastising Gov. Ron DeSantis of Florida over his super PAC’s disclosure of debate strategy memos, which called for him to defend Mr. Trump against Mr. Christie’s attacks onstage.

If Mr. DeSantis ends up doing so, Mr. Christie said he had one piece of advice: “Get the hell out of the race.”

In an interview this month during his surprise visit to Ukraine, Mr. Christie said he was not particularly bothered by the prospect of a debate without Mr. Trump.

“It doesn’t change my perspective or my tactical approach,” he said. “Because if he’s not there, it just means two things. One, he’s afraid to be on the same debate stage and defend his record. And two, you know, this is a guy who, by not showing up, just gives me more time. So it’s OK. Either way, I win.”

Mr. Christie and his advisers see the debate as a forum built to the former governor’s strengths. Comfortable in unscripted moments in front of cameras, Mr. Christie has a confrontational style, hewed from years in the trenches of New Jersey politics, that has served him well in past debates, and he has over a decade of experience participating in them through two campaigns for governor and his 2016 run for president.

To prepare for Wednesday, he has been huddling with close advisers to go over topics at the heart of the campaign and anticipate different scenarios that may arise as he navigates the chaos of an eight-lectern stage. He has been heavily focused on the debate, bringing it up in casual conversations with both advisers and political acquaintances as he takes the temperature of the race.

At the same time, his preparations have a bare-bones nature. There are no mock debates, no fake stages with podiums, no advisers suiting up for the roles of Mr. Trump or Mr. DeSantis.

In part, he is informed by his experience during the 2016 presidential debates, when he notably avoided attacking Mr. Trump. (He has said on the campaign trail that he was the only candidate to go speak to Mr. Trump during commercial breaks.) Mr. Christie was quick to pounce on his other rivals, including a now-famous exchange with Senator Marco Rubio of Florida.

“There it is,” Mr. Christie said, interrupting Mr. Rubio, who had pivoted to a line about former President Barack Obama during an exchange with Mr. Christie. “The memorized 25-second speech. There it is, everybody.”

That dismissive riposte sent the Rubio campaign spiraling, with headlines concluding that Mr. Christie had exposed Mr. Rubio as a robotic candidate reliant on consultants and that the Florida senator had “choked.” After polling near second or third place in New Hampshire before the debate, Mr. Rubio finished fifth in the state’s primary race less than a week later.

Built into the question of how Mr. Christie treats this week’s debate is just how much Republican voters want to see someone caustically rip into Mr. Trump, whether he is onstage or not.

Despite his mounting legal problems, Mr. Trump remains exceptionally popular within the party. And Mr. Christie’s constant provocations, beyond endearing the former governor to some moderate Republicans, have also turned him into something of a #Resistance hero among liberals who will not be voting in a G.O.P. primary.

Waiting for a flight at Kennedy International Airport in New York early this month, Mr. Christie was approached for a photo by a fellow traveler, Jessica Rutherford, who told him she appreciated his broadsides against Mr. Trump and hoped he would continue.

“You’re like Obi-Wan Kenobi, you’re our only hope!” she told him.

But Ms. Rutherford, an intellectual property lawyer from Wilton, Conn., and a Democrat, conceded that she was unlikely to vote for Mr. Christie in November 2024 if he were to win the Republican primary.

Undaunted, Mr. Christie made his pitch. “I’ll be awake in meetings with foreign leaders,” he offered, in a jab at President Biden’s age.

“Reagan wasn’t awake in meetings with foreign leaders,” Ms. Rutherford shot back.

“I bet you didn’t vote for him, either,” Mr. Christie replied.

Patricia Mazzei contributed reporting from South Miami, Fla.