The candidates largely swapped only glancing blows in the early nominating contests in Iowa and New Hampshire in January, but the rhetorical artillery fire has intensified since the primary narrowed into a two-horse race.
"Tomorrow you will cast one of the most important votes of your entire life and -- honestly -- we're not very worried about tomorrow," a nonchalant Trump told an election-eve rally in the city of Rock Hill.
Seeking to demonstrate that he was already looking beyond Haley, he vowed to show President Joe Biden and the Democrats "that we are coming like a freight train in November," when the general election will be held.
Polls in the southern US state opened at 7:00 am local time (1200 GMT). In a school near Charleston, about a dozen people came to cast votes in the first half hour.
South Carolinians do not have to indicate party allegiance when they register to vote, and are allowed to have their say in either the Democratic or the Republican primary.
Haley -- a more traditional conservative who espouses limited government and a muscular foreign policy -- will rely on votes from moderates, although the tactic did little for her as she lost to Trump in each of the first four nominating contests.
- Legal fees -
Voters interviewed by AFP in South Carolina capital Columbia on Thursday were complimentary about both candidates, although one voter felt Haley wasn't ready for the highest office and another criticized Trump for being "divisive."
"He'll go after people that don't agree with him. Being a Christian, I don't feel good about that," said financial advisor and Haley voter David Gilliam, 55.
The primary comes amid signs that the frontrunner -- who faces four criminal indictments -- is tightening his hold over the party as he pushes for a reshuffle to install family members and allies at the top of the Republican National Committee.
His daughter-in-law Lara Trump has promised to spend "every single penny" of party funds on his presidential campaign should she become an RNC cochair, and has argued that paying his legal bills is of "big interest" to Republican voters.
Haley has sought to focus on the "chaos" that she says follows Trump, pointing to $8 million in campaign donations he spent on legal fees in January and predicting that his total outlay on court cases this year could top $100 million.
- IVF ruling -
"He has turned his presidential campaign into a legal defense slush fund and will not have the resources or focus to go up against Joe Biden and the Democrats," said Haley national spokeswoman Olivia Perez-Cubas.
In common with Democrats, Haley has also been hitting Trump over his outlook on the international stage and oft-voiced admiration for the leaders of the world's most authoritarian regimes.
She has blasted Trump's reaction to the death of Russian dissident Alexei Navalny -- in which he avoided criticism of President Vladimir Putin -- and his threat to encourage Moscow to attack NATO nations that had not met their financial obligations.
But Haley's central argument for months has been that polling shows her performing better than Trump in hypothetical matchups with Biden.
She has vowed to compete in the Republican primary through "Super Tuesday" -- when multiple states vote on March 5 -- regardless of what happens in South Carolina on Saturday.
Reproductive rights are likely to figure prominently in the election, with Trump avoiding taking a clear position on proposals for a nationwide abortion ban after appointing three Supreme Court justices who helped gut federal protections.
A wrinkle was added when Alabama's supreme court ruled last week that frozen embryos can be considered children, signaling a new front in the debate and posing questions for in vitro fertilization (IVF) clinics.
Trump -- keenly aware the Alabama decision risks alienating moderate and women voters -- voiced support Friday for preserving access to IVF programs, calling on the state's legislature to "act quickly to find an immediate solution" to ensure it remained available.
