She plans to say in her speech that the onus is on Mr. Trump to win over the voters who supported her, the people said.

For months, Ms. Haley, who was Mr. Trump’s first ambassador to the United Nations, had tried to paint her former boss as an aging, mentally unsound agent of chaos, unable to respect veterans or service members and unwilling to be faithful to the Constitution.

But even as she amplified her attacks, she was never able to sufficiently loosen Mr. Trump’s grip on the party. Instead, she absorbed loss after loss, including in her home state late last month. That streak culminated in Mr. Trump’s Super Tuesday blowout. Of the 15 states that voted, Ms. Haley won only Vermont, to add to her victory Sunday in the District of Columbia.

On the Democratic side, President Biden swept all 15 states that held Democratic contests, as well as the Iowa caucuses. His one stumble came in American Samoa, a tiny American territory in the Pacific Ocean, where he tied a little-known businessman, Jason Palmer, for the territory’s delegates, three to three, according to The Associated Press.

Ms. Haley’s campaign did highlight fractures in the Republican electorate — she consistently won enough votes in enough states to raise questions about Mr. Trump’s viability in November. But her argument that only she could beat Mr. Biden was undercut by polls showing him doing so.

She faced intense pressure from Republican Party officials who feared that a post-Super Tuesday campaign would further divide the G.O.P. and help Mr. Biden.

Speaking at his Palm Beach, Fla., home on Tuesday night — before the news that Ms. Haley would drop out — Mr. Trump did not mention her, instead calling for “unity.”

Ms. Haley, at first, was having none of it. Her campaign spokeswoman, Olivia Perez-Cubas, fired off a statement saying: “Unity is not achieved by simply claiming, ‘We’re united.’ Today, in state after state, there remains a large block of Republican primary voters who are expressing deep concerns about Donald Trump.”

Behind Ms. Haley’s anti-Trump rhetoric over the past few months was a notably cautious and conventional campaign. She repeated her stump speech almost word for word and rarely took questions from voters or the news media.

Ultimately, she failed to stitch together a coalition of Republicans who liked Mr. Trump’s policies but had wearied of his baggage, were drawn to her call to return the party to fiscal responsibility and international leadership or were eager to leave the Trump era behind.

The presidential candidates were not the only ones on the ballot on Tuesday. Here’s what else happened:

• In North Carolina, the same Republican voters who sided with Mr. Trump selected Mark Robinson — the state’s conservative lieutenant governor, who has a history of offensive and polarizing comments — to run for governor in November. Democrats selected the mild-mannered, popular state attorney general, Josh Stein, to run against Mr. Robinson in a contest to succeed North Carolina’s term-limited Democratic governor, Roy Cooper.

• In California, where voters pick two top finishers to face off in November regardless of party, the race for Dianne Feinstein’s Senate seat will be between Representative Adam Schiff, a Democrat, and the former Los Angeles Dodgers great Steve Garvey, a Republican. Mr. Schiff, hoping to square off with Mr. Garvey in heavily Democratic California, had run advertisements declaring him “too conservative” for California, an attack that elevated Mr. Garvey’s almost invisible campaign. That knocked out Representative Katie Porter, a Democrat who would have been a more formidable challenger in the fall.