Incumbent US President Joe Biden has secured a decisive win in the Democratic presidential primary in the crucial state of South Carolina, notching his first official victory in his largely unobstructed path to become his party's nominee for a possible rematch against Donald Trump in the November 5 election.

It was the first official primary of the ruling party. South Carolina is the state that launched Biden to the Democratic nomination four years ago.

Biden, 81, who is seeking re-election in the November presidential elections, received 96.2 per cent of the votes in Saturday's primary, with almost non-existent Marianne Williamson getting just 2.1 per cent of the votes.

Biden's decisive win, which was called almost immediately after polls closed, was not a surprise given his lack of a formidable challenger.

Biden, 81, faces concerns within his party about his age and ability to fire up young voters. Yet despite his flaws as a candidate, a viable Democratic challenger never emerged last year.

The Republican primary in the state is scheduled to be held on February 24. Former President Trump, 77, is the favourite in that race.

“As I said four years ago, this campaign is for everyone who has been knocked down, counted out and left behind. That is still true today. With more than 14 million new jobs and a record 24 straight months — two years — of the unemployment rate under 4%, including a record low unemployment rate for Black Americans, we are leaving no one behind,” Biden said in a statement.

He said he would make Republican rival Trump a "loser" in November's election.

“In 2020, it was the voters of South Carolina who proved the pundits wrong, breathed new life into our campaign, and set us on the path to winning the Presidency. Now in 2024, the people of South Carolina have spoken again and I have no doubt that you have set us on the path to winning the Presidency again — and making Donald Trump a loser — again,” he said.

Observing that the stakes in the election could not be higher, Biden said there are extreme and dangerous voices at work in the country — led by Trump — who are determined to divide the nation and take it backwards.

“We cannot let that happen. We’ve come a long way these past four years — with America now having the strongest economy in the world and among the lowest inflation of any major economy. Let’s keep pushing forward. Let’s finish what we started — together,” Biden said.

Democrats approved a shake-up of their 2024 primary calendar last year and made South Carolina their first official primary state after Biden argued the new nominating order would better reflect the diversity of the nation and the party.

Biden had travelled to South Carolina twice in January. First Lady Dr Jill Biden and Vice President Kamala Harris also campaigned in the State.

South Carolina is unlikely to be a battleground State, but the victory is likely to energise the Democratic Party and Biden's base across the country.

Soon after the decisive victory, Biden-Harris 2024 communications director Michael Tyler took to the airwaves to highlight how this primary did exactly what Biden called for elevating Black voters and giving them the first say in the Democrats’ nominating process.

“We're going to talk about Donald Trump, who, when he was president, we saw the damage he caused to Black America," he said.

"The only things that went up under Donald Trump's presidency for Black folks were the unemployment rate and the uninsured rate. His tax cut, for example, gave double the benefit to the typical white household than the typical Black household,” he told MSNBC.

Earlier, Biden easily won an unusual Democratic write-in vote in New Hampshire's primary election in January, even though he was not on the ballot.

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