The former president had been widely favored to win the Southern state, with one opinion poll after another showing him holding a sizable lead despite his litany of criminal charges and Haley's status as a native of South Carolina who won two terms as governor.

"I have never seen the Republican party so unified as it is right now," Trump told supporters in Columbia, the state capital, just minutes after the polls closed at 7 p.m. He did not mention Haley once in about 30 minutes of remarks.

The lopsided outcome will bolster calls from Trump's allies that Haley, Trump's last remaining challenger, should drop out of the race. Trump has dominated all five contests thus far - in Iowa, New Hampshire, Nevada, the US Virgin Islands and now Haley's home state - leaving her with virtually no path to the Republican nomination.

A defiant Haley, who served as UN ambassador under Trump, insisted this week that she would sustain her campaign through at least "Super Tuesday" on March 5, when Republicans in 15 states and one US territory will cast ballots.

It was too early in the evening to know whether Haley would capture a stronger-than-expected share of the vote, which could allow her to argue that she has some momentum heading toward Super Tuesday. Trump was leading 63 percent to 36.8 percent with just 2.5 percent of the expected vote tallied, according to Edison.

She has notably sharpened her attacks on Trump in recent days, questioning his mental acuity and warning voters that he would lose November's general election.

But there is scant evidence that Republican voters are interested in any standard-bearer except Trump.

Immigration, which Trump has made a key focus of his election campaign, was the No. 1 issue for voters in the Republican primary on Saturday, according to an Edison exit poll. Some 39 percent of voters cited that issue compared with 33 percent who said the economy was their top concern.

Approximately 84 percent of voters said the economy is not so good or poor, highlighting a major potential weakness for Biden in November's general election.

Once again, however, exit polls also pointed to Trump's own vulnerabilities. Nearly one-third of voters on Saturday said Trump would be unfit to serve as president if he were convicted of a crime.

Trump's first criminal trial is scheduled to begin on March 25 in New York City. He is charged in that case with falsifying business records to conceal hush money payments made to porn star Stormy Daniels.

He faces three other sets of charges, including a federal indictment for conspiring to reverse Biden's victory in 2020. Trump has pleaded not guilty in every case and claimed, with no evidence, that the charges stem from a Democratic conspiracy to derail his campaign.

'My ultimate and absolute revenge'

Both Trump and Biden have already begun looking ahead to November, with the president characterizing Trump as a mortal threat to the republic.

Before flying to South Carolina to watch returns on Saturday, Trump addressed a gathering of conservative activists near Washington in a 90-minute speech that painted a dark picture of a declining America under Biden.

He said if he beats Biden in the Nov. 5 general election it will represent a "judgment day" for the US, and "my ultimate and absolute revenge."

South Dakota Governor Kristi Noem and former presidential candidate Vivek Ramaswamy emerged as favorites for Trump's vice presidential pick, according to a poll of activists at the conservative conference. They each received 15 percent support.

Haley, whose foreign policy credentials are at the center of her campaign, has focused in recent days on Trump's stance toward Russia following the death of Alexei Navalny, the main opposition leader there.

She criticized Trump for waiting days before commenting on Navalny's death and then for failing to blame Russian President Vladimir Putin. She also condemned Trump's recent remarks that he would not defend NATO allies from a Russian attack if he felt they had not spent enough on defense.

Haley had hoped that South Carolina's "open" primary, which allows registered voter to cast a ballot, would lead to turnout among independents and even some Democrats who are determined to stop Trump.

But Edison exit poll data showed only 21 percent of voters considered themselves moderate or liberal, only slightly higher than the 19 percent who said the same in the party's 2016 primary.

Kelli Poindexter, a Democrat and transcriptionist who lives in Columbia, voted for Haley "simply to, maybe, cancel out one of the Donald Trump votes."

"I think he's dangerous," Poindexter said. "I think he's a threat. And if Democrats come out and give a vote to Nikki, it takes one away from him."

But Kevin Marsh, a 59-year-old Republican and truck driver who also lives in Columbia, said he voted for Trump on Saturday because he trusts him more than Haley.

"She's more of a globalist and I just can't support that," Marsh said.

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