Former President Donald J. Trump easily defeated Nikki Haley in South Carolina’s Republican primary on Saturday, delivering a crushing blow in her home state and casting grave doubt on her long-term viability.

Mr. Trump’s victory, called by The Associated Press, was widely expected, and offers fresh fodder for his contention that the race is effectively over. Ms. Haley pledged to continue her campaign, but the former president has swept the early states and is barreling toward the nomination even as a majority of delegates have yet to be awarded.

“This was a little sooner than we anticipated,” he said in Columbia, S.C., minutes after the race was called, adding that he had “never seen the Republican Party so unified as it is right now.”

Throughout his victory speech, Mr. Trump made it clear that he was eager to turn his attention to the general election, at one point telling the crowd: “I just wish we could do it quicker. Nine months is a long time.”

He also did not mention Ms. Haley by name, alluding to her only twice: once to knock her for a disappointing finish in a Nevada primary contest with no practical value, and once for supporting an opponent of his in 2016.

In her election-night speech in Charleston, S.C., Ms. Haley congratulated Mr. Trump on his victory. But she said the results — which showed her with about 40 percent of the vote when she spoke around 8:30 p.m. — demonstrated that “huge numbers of voters” were “saying they want an alternative.”

Mr. Trump, however, won South Carolina in 2016 and has remained popular in the state since, with polls ahead of the primary consistently showing him with double-digit leads.

Ms. Haley, the former governor of South Carolina and a United Nations ambassador during Mr. Trump’s administration, had hoped to buck the odds, but her loss at the hands of voters who are arguably the most familiar with her politics will fuel further uncertainty about her path forward.

During her speech, Ms. Haley sounded more serious and less upbeat than she had after defeats in Iowa and New Hampshire. But she said she planned to stay in the race through Super Tuesday on March 5, arguing that Americans deserved a chance to choose a candidate.

“In the next 10 days, another 21 states and territories will speak,” she told supporters. “They have the right to a real choice. Not a Soviet-style election with only one candidate.”

Ms. Haley has staked her campaign on drawing support from independents and more moderate Republicans, particularly in states where primaries are not restricted to voters registered with one party.

But that strategy fell short in New Hampshire last month — the early-voting state where she was closest to Mr. Trump in polls — and in South Carolina, raising questions about whether it will succeed in Michigan, which holds its primary on Tuesday, and any of the 16 states that vote on Super Tuesday on March 5.

Still, Ms. Haley has insisted she will stay in the race, arguing that she is providing an alternative for voters opposed to Mr. Trump and maintaining that Americans deserve a chance to choose a candidate.

So far, though, Republican voters have shown no sign of turning away from Mr. Trump, even as he faces 91 felony charges in four criminal cases. Mr. Trump’s legal problems have been at the forefront of his bid, as he tries to use the unprecedented collision between the campaign trail and courtrooms to rally his base behind him.

Mr. Trump’s first criminal trial, on charges connected to a hush-money payment to a porn star in 2016, is scheduled to start on March 25 in New York City, meaning his trial could overlap with dozens of Republican primaries and caucuses.

Whether Ms. Haley will remain in the race by then is an open question. Donors have so far continued to pour money into her bid, giving her the cash to keep going. She will travel to Michigan on Sunday and has planned stops in a number of states before the Super Tuesday contests, when 36 percent of Republican delegates will be up for grabs.

“We don’t anoint kings in this country,” Ms. Haley said on Tuesday. “We have elections. And Donald Trump, of all people, should know we don’t rig elections.”

The Trump campaign has repeatedly signaled its desire to focus on the general election and an anticipated matchup against President Biden, who won South Carolina’s Democratic primary early this month.

In a speech earlier on Saturday at the Conservative Political Action Conference outside Washington, Mr. Trump focused entirely on Mr. Biden rather than addressing Ms. Haley, his more immediate opponent.

Mr. Trump and his team have called on Ms. Haley to drop out of the race, pointing to his delegate tally and his lead in polls as proof that she has no mathematical path to the nomination.

Mr. Trump’s followers have outnumbered Ms. Haley’s at every turn of the contest so far. Even in Nevada, where Ms. Haley was the only candidate in a Republican primary that awarded no delegates, she was outvoted by a “None of These Candidates” option on the ballot. Ms. Haley did not campaign there and her campaign shrugged off the symbolic defeat, but it generated days of embarrassing headlines.

Over the last month, Trump advisers have taken every opportunity to argue that Ms. Haley has yet to name a state whose primary she thinks she can win. Mr. Trump sought to undercut and humiliate her well ahead of South Carolina.

In New Hampshire, the Trump campaign showcased her relative lack of support at home by bringing a slew of prominent South Carolina Republicans to the state, including Gov. Henry McMaster and Senator Tim Scott, whom Ms. Haley appointed to his position.

Both men have appeared regularly at Mr. Trump’s South Carolina rallies, with Mr. Scott, a former rival for the Republican nomination, emerging as a key surrogate and a potential running mate. Mr. Trump has also begun to claim that he only tapped Ms. Haley for the United Nations post in his administration in order to clear the way for Mr. McMaster to become governor.

That line is part of an increasingly aggressive barrage of attacks that Mr. Trump has unleashed at Ms. Haley since the Republican field narrowed. After earlier only criticizing Ms. Haley’s standing in the polls, he began taking aim at her political views while lobbing personal smears about her temperament, intelligence and marriage.

Ms. Haley, for her part, has also leveled sharp critiques at Mr. Trump, building on her monthslong argument that Republicans need a younger leader who can leave behind the “chaos” of the Trump era. She has called him “unhinged” and suggested that he would use the Republican National Committee’s coffers to pay his mounting legal bills as he fights his criminal indictments.

Her loss in South Carolina marked a striking political transformation for both her and the Republican Party. When Ms. Haley ran for governor in 2010, she was the anti-establishment candidate embraced by grass-roots conservatives aligned with the Tea Party who saw her as an outsider.

But the movement that propelled her success coalesced behind Mr. Trump in 2016, helping him dominate Republican politics and remake the party in his image. Ms. Haley, once seen as being on the party’s conservative fringes, now appears to be too moderate for the Republican base.

Jazmine Ulloa contributed reporting.