Donald Trump easily defeated Nikki Haley in South Carolina's Republican contest on Saturday, Edison Research projected, extending his winning streak as he marches toward a third consecutive presidential nomination and a rematch with Democratic President Joe Biden.

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 pm (0000 GMT). 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% to 36.8% with just 2.5% 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% of voters cited that issue compared with 33% who said the economy was their top concern.

Approximately 84% 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.