South Carolina have all but ordained Donald Trump as the once and future presidential candidate by handing him a sweeping primary victory over Nikki Haley, his last remaining challenger.

Trump has now swept every contest that counted for Republican delegates, with wins already in Iowa, New Hampshire, Nevada and the US Virgin Islands.

He had been widely favoured 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.

With counting underwat late on Saturday night (US time),  Trump had secured 61 per cent of votes, 22 points ahead of Haley.

“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.

He added: “You can celebrate for about 15 minutes, but then we have to get back to work.”

He did not mention Haley once in about 30 minutes of remarks.

Precedents and omens

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 leaving her with no path to the Republican nomination.

South Carolina’s primary has historically been a reliable bellwether for Republicans. In all but one primary since 1980, the Republican winner in South Carolina has gone on to be the party’s nominee. The lone exception was Newt Gingrich in 2012.

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 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 59.6 per cent to 39.7 per cent with 50 per cent of the vote tallied, according to Edison research.

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.

Border crisis

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 per cent of voters cited that issue compared with 33 per cent who said the economy was their top concern.

Approximately 84 per cent 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 that the charges stem from a Democratic conspiracy to derail his campaign.

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

-AAP