Donald Trump and Nikki Haley go head-to-head on Saturday in South Carolina's Republican primary, with the ex-president set to trounce his former charge in her home state as he closes in on the nomination.

Haley was a popular governor of the state before becoming Trump's UN ambassador in 2017, but her old boss is backed by the local party establishment and nearly two-thirds of voters in opinion polling.

Since the early nominating contests in Iowa and New Hampshire in January, the rhetorical fire has intensified between the two as the primary narrowed into a two-horse race.

But seeking to demonstrate that he is looking beyond Haley, Trump has 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.

In remarks released early to the US media, Trump will say in a speech later on Saturday that "if we can break out of this Biden nightmare, we have it in our grasp to make America richer, safer, stronger, prouder and more beautiful than ever before."

Polls in the southern US state opened at 7:00 am local time (1200 GMT).

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 hope for votes from moderates, but the tactic did little for her as she lost to Trump in each of the first four nominating contests. (AFP)