COLUMBIA, South Carolina, Feb 24 (Reuters) - Donald Trump easily defeated Nikki Haley in South Carolina's Republican contest on Saturday, 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, despite his litany of criminal charges and Haley's status as a native of South Carolina who won two terms as governor.

The big win will bolster calls from Trump's allies that Haley, Trump's last remaining challenger, should drop out of the race. But Haley, who appeared to have outperformed expectations based on opinion polls, again defiantly insisted she would fight on at least through "Super Tuesday" on March 5, when Republicans in 15 states and one U.S. territory will cast ballots.

Trump was leading 60.0% to 39.4%, a margin of 20.6 percentage points, with about 83% of the expected vote tallied, according to Edison. Statewide opinion polls prior to Saturday had given Trump an average lead of 27.6 percentage points, according to the tracking website 538.

"Forty percent is not some tiny group," Haley said, referring to her approximate vote share on Saturday. "There are huge numbers of voters in our Republican primaries who are saying they want an alternative."

Trump has now dominated all five contests thus far - in Iowa, New Hampshire, Nevada, the U.S. Virgin Islands and now Haley's home state - leaving Haley with no evident path to the Republican nomination.

Trump gave his victory speech in Columbia, the state capital just minutes after the polls closed at 7 p.m. EST (0000 GMT), and didn't mention Haley once, claiming his party's mantle as he looked ahead to November's general election.

"I have never seen the Republican Party so unified as it is right now," he said.

Haley notably sharpened her attacks on Trump in recent days, questioning his mental acuity and warning voters that he would lose the general election.

But there is scant evidence that a majority of Republican voters is 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.

Photo from Reuters