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

Minutes before his announcement in Windham, N.H., Mr. Christie 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.”

In his speech, Mr. Christie did not endorse any of his rivals, nor did he address their prospects against Mr. Trump, dashing the hopes of Republican moderates that his exit would unify remaining members of the party who oppose Mr. Trump.

In fact, Mr. Christie denounced his opponents’ 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, which came after mounting pressure from within his party, effectively ends a phase of the Republican presidential contest, removing from the field its most aggressive Trump critic. He was the only prominent contender who declared that Mr. Trump was unfit for office — an argument that all but doomed his candidacy from the start.

Nonetheless, Mr. Christie used the final moments of his campaign to unleash one last extended criticism of Mr. Trump, eviscerating his policies, lamenting the direction in which he has taken the country and asserting that he did not have the nation’s best interests at heart.

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

His speech doubled as a dark warning for a party — and a country — that the former governor portrayed as veering dangerously off course, criticizing “the hate and the division and the selfishness of what our party has become under Donald Trump.”

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,” he said. “Those questions don’t ever leave. In fact, they’re really stubborn. They stay.”

Despite Mr. Christie’s withering criticisms of his rivals, his decision could turn the primary election in New Hampshire on Jan. 23 into a two-person race between Mr. Trump and Ms. Haley. Her positions on foreign policy, national security and the rule of law broadly overlap with Mr. Christie’s, and she will hope to consolidate never-Trump Republicans and independents behind her.

After Mr. Christie’s speech, Ms. Haley praised him as “a friend for many years,” commending him in a statement “on a hard-fought campaign” but making no reference to the hot-mic comments. “I will fight to earn every vote,” she said.

On Wednesday, Mr. DeSantis had called Mr. Christie to express his appreciation for his role in the contest, according to two people with knowledge of the call. During their conversation, Mr. Christie mocked Ms. Haley and said she was not up to the task, the people said.

Mr. DeSantis wrote on social media on Wednesday, “I agree with Christie that Nikki Haley is ‘going to get smoked.’”

Recent polls have shown Ms. Haley narrowing the gap against Mr. Trump in New Hampshire, and her backing combined with Mr. Christie’s support has sometimes equaled or bettered the former president’s. A CNN/University of New Hampshire poll released Tuesday found Mr. Trump with 39 percent support, Ms. Haley with 32 percent and Mr. Christie with 12 percent.

Mr. Christie, a former United States attorney, built his candidacy around a prosecutorial argument about his domineering rival’s unsuitability for office. He steadfastly refused to water down his denunciations of Mr. Trump, a onetime ally turned bitter antagonist, even as most of his rivals labored to find a middle ground of praise and subtle contrasts.

That bold stance, and Republican voters’ lack of tolerance for it, left Mr. Christie trailing far behind in polls and fund-raising, managing to grab a foothold only in independent-minded New Hampshire. Yet the former governor regularly found himself in the shadow of Ms. Haley in the Granite State during the closing months of the campaign, with Gov. Chris Sununu endorsing Ms. Haley in December and later calling on Mr. Christie to withdraw from the race.

For weeks, Mr. Christie rebuffed any suggestion that he should drop out, stressing the argument that his role as Mr. Trump’s chief critic in a dwindling Republican field was vital.

As recently as Tuesday, he spoke at length about his reasons for forging ahead.

“Let’s say I dropped out of the race right now, and I supported Nikki Haley,” he said. “And then three months from now, four months from now, we get ready to go to the convention. She comes out and is his vice president. What would it look like? What will all the people who supported her at my behest look like when she’s up on a stage in Milwaukee with her hands up like this with Donald Trump?”

Mr. Christie’s familiarity with the former president gave him endless ammunition for attacks. He mocked Mr. Trump’s taste for well-done hamburgers and his love of cable news. He called attention to Mr. Trump’s criminal indictments, even as other Republican candidates sought to support the former president through his legal woes.

But despite this barrage, Mr. Christie’s condemnation of Mr. Trump never took hold. He made a cringeworthy attempt to nickname his rival “Donald Duck” over Mr. Trump’s ducking of the primary debates. He was drowned out with resounding boos during the first debate when he tried to criticize Mr. Trump.

In an attempt to show both expertise in foreign policy and a fearlessness to lead, Mr. Christie embarked on trips to conflict zones in Ukraine and Israel. He vividly recounted atrocities he had learned about in each place to buttress his aggressive support of those countries, though neither overseas venture broke through with voters.

As he campaigned, Mr. Christie became more moderate than he had been in the past, either admitting to mistakes like endorsing Mr. Trump or evolving on issues such as same-sex marriage. “I was wrong,” Mr. Christie said of his opposition to such marriages at an event in New Hampshire last month. He peppered his stump speeches with references to former President George W. Bush, hardly a popular figure with today’s Republican base.

Betting his entire campaign on New Hampshire, Mr. Christie argued in an interview with The New York Times in September that “once Donald Trump loses in one place, that entire rotted building will crumble.” He held dozens of town halls and other events across the small state, building modest traction and rising in polls, though he never came within striking distance of Mr. Trump. Mr. Christie’s allied super PAC dumped all of its advertising budget — $5.1 million as of Wednesday, according to AdImpact, a media-tracking firm — into the state.

Some of Mr. Christie’s supporters expressed disappointment after his event on Wednesday. Toni Pappas, a Hillsborough County commissioner from Manchester, N.H., was one of them, but said, “I think he did something very noble and patriotic.”

Tom Barton, a self-described libertarian from Washington, N.H., who planned to vote for Mr. Christie, said he could not see himself supporting another Republican. “They don’t have the courage to tell the truth about Trump,” he said.

Still, most Republican voters had remained firmly opposed to Mr. Christie, who trudged on without changing his approach. If anything, his resolve to attack the former president increased.

“The future of this country is going to be determined here,” Mr. Christie told a crowd at a New Hampshire brewery in September, clutching an I.P.A. It was a warning he would issue at nearly every campaign stop. “If Donald Trump wins here, he will be our nominee. Everything that happens after that is going to be on our party and on our country. It’s up to you.”

Shane Goldmacher, Neil Vigdor, Maggie Astor and Alyce McFadden contributed reporting.

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