Republican voters in the US state of South Carolina were casting ballots Saturday in a primary that could be key in determining whether Donald Trump can push forward to become the centre-right party's presidential candidate.

Meanwhile, Democrats were set to hold a caucus at 1800 GMT Saturday in Nevada pitting former secretary of state Hillary Clinton against Vermont Senator Bernie Sanders.

There was a steady flow of voters at a municipal centre in suburban Mt Pleasant, South Carolina on Saturday morning with voters expressing their concerns about the economy and Islamic State among other issues.

Trump supporters expressed anger at a broken political system and the influence of money in politics, while other voters told dpa they had chosen to back Marco Rubio, whom they see as a fresh, electable candidate.

Trump "can't be bought and he'll tell you to your face what he thinks and how he feels," said Michael Reilly, 18, who was voting in his first election.

Rubio supporter Kurt Hiott, 29, said the Florida senator would be a "good face for the conservative movement" who could attract Democrats and independents in November general elections.

Opinion surveys in recent weeks have shown Trump with a substantial lead in South Carolina, and he has drawn large crowds as he crisscrosses the south-eastern state, but an NBC/Wall Street Journal poll released Friday showed his lead had shrunk to just 5 percentage points ahead of Texas Senator Ted Cruz.

The candidates trailing Trump hope to narrow the field and emerge as the alternative to the real estate tycoon, whose brash campaign style has equally alienated and energized voters. The first two contests in Iowa and New Hampshire saw wins for Cruz and Trump, respectively.

Trump stood at 28 per cent support to 23 per cent for Cruz in South Carolina, the poll showed. Rubio had 15 per cent, former Florida governor Bush 13 per cent and 9 per cent each for Ohio Governor John Kasich and retired neurosurgeon Ben Carson.

South Carolina, which is only the third in the state-by-state process to chose presidential candidates, is known for its bare-knuckled political style, and television and radio ads airing in the state feature the candidates slinging mud at each other on a variety of issues. Trump accused rival Cruz of being an untrustworthy politician, and Bush bashed Trump's crude language.

In Nevada, Clinton had once held a substantial lead bolstered by the western state's large Hispanic population, but has seen her advantage slip after Sanders notched a large victory in New Hampshire and ended in a near tie in Iowa.

A CNN opinion poll released this week showed Clinton with 48 per cent to Sanders' 47 per cent.

The Clinton campaign had stressed she was better positioned to gain support of the minority voters who are a key Democratic voting bloc.

Her campaign has described Nevada, South Carolina and southern states with primaries next month as her "firewall" on the path to the nomination, but now could be in trouble as Sanders, a self-described democratic socialist, has seen his message of economic equality resonate with voters.

The Democrats next hold their own primaries in South Carolina on February 27 and Republican hold caucuses in Nevada on Tuesday.

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