Trump is overwhelmingly favored to win the Southern U.S. state's primary election, the fifth Republican nominating vote in a campaign the former president has dominated from the outset despite facing dozens of criminal charges.
He swept the first four contests in Iowa, New Hampshire, Nevada and the U.S. Virgin Islands, knocking out the rest of the initially large Republican field along the way.
Opinion polls show Trump holding an average statewide lead of 30 percentage points, according to the poll tracking website 538. The Republican nominee will face Democratic President Joe Biden in the Nov. 5 election.
Haley, a native of South Carolina who served as its governor from 2011 to 2017, dismissed suggestions this week that a home-field defeat would prompt her to end her White House bid, even though she does not have a clear path to the nomination.
She has vowed to press on to Super Tuesday in early March, when 15 states and one U.S. territory will vote, including Texas, Virginia and North Carolina, awarding around one-third of the delegates to July's Republican National Convention, which will choose the nominee.
But a lopsided victory for Trump, 77, on Saturday would only increase pressure on Haley, 52, to drop out so that the former president can turn his attention to campaigning against Biden. The president is already treating Trump as the Republican nominee and painting him as a mortal threat to the republic.
Polls will be open on Saturday from 7 a.m. to 7 p.m. ET (1200 to 0000 GMT).
South Carolina's primary is "open," allowing any registered voter to participate, which could offer Haley a boost if independents and Democrats - who tend to favor her over Trump - turn out in force.
In contrast to the campaign's early stages, Haley has sharply attacked Trump leading up to Saturday's vote, warning Republican voters that a third consecutive Trump nomination will end in defeat.
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.
Trump has questioned Haley's intelligence, nicknaming her "birdbrain," and frequently reminds voters that the state's top Republicans, including Governor Henry McMaster, have endorsed his campaign.
"You're not supposed to lose your home state," Trump told a Fox News town hall this week.
A New York native, Trump lost the state but won the overall election in 2016. He moved his permanent residence to Florida in 2019.
With a massive lead in national opinion polls, Trump could effectively clinch the nomination by mid-March if he wins primaries at the same pace - just in time for his first criminal trial, which 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.
