Former US President Donald Trump went into Saturday's Republican primary in South Carolina as the front-runner and emerged victorious over the state's former governor Nikki Haley.

The Associated Press declared Trump the winner as polls closed at 7 p.m. EST, based on a survey of Republican voters in the state. Projections by CNN and NBC also declared Trump the winner.

Trump gained approximately 60% of the vote, while Haley received some 40%, according to pollster Edison Research.

"I have never seen the Republican Party so unified as it is right now," Trump said in a victory speech delivered moments after polls closed.

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

Despite losing in her home state, Haley has vowed to stay in the race.

"I said earlier this week that no matter what happens in South Carolina, I would continue to run ... I'm a woman of my word. I'm not giving up this fight when a majority of Americans disapprove of both Donald Trump and Joe Biden," she said after the vote.

"Forty percent is not some tiny group," Haley added. "There are huge numbers of voters in our Republican primaries who are saying they want an alternative."

Trump closes on on Republican nomination

The result is Trump's fifth straight win during primary season after victories in Iowa, New Hampshire, Nevada and the US Virgin Islands.

David Darmofal, a politics professor at the University of South Carolina, told the AFP news agency that Trump's clear victory on Friday night made him "effectively the presumptive Republican nominee for president."

President Joe Biden also reacted to the result with a statement warning voters of "the threat Donald Trump poses to our future as Americans grapple with the damage he left behind."

Voters unswayed by Trump's legal battles

Haley spent much of the campaign in her home state pointing out that 77-year-old Trump faces four criminal indictments.

But nearly two-thirds (65%) of Republican primary voters said Trump would still be fit for the presidency if he were convicted of a crime, according to exit polls.

The same percentage of those who voted said they do not think US President Joe Biden legitimately won the presidency in 2020, the polls showed.

Rather than the charges against Trump, voters told pollsters they were most concerned about immigration (41%) and the economy (31%). Abortion policy was most important to 10% of primary voters and foreign policy was the top issue for 11%, according to exit polls.

zc/sms (AP, Reuters, AFP)  

Correction: An earlier version of this article erroneously stated that Joe Biden won the election in 2016. We apologize for the mistake, which ahs now been corrected.