Taking aim at Sanders, who serves as an independent in the Senate, Biden said: "If the Democrats want a nominee who's a Democrat - a lifelong Democrat, a proud Democrat, an Obama-Biden Democrat - then join us!"

He continued: "Most Americans don't want the promise of revolution. They want more than promises: they want results."

South Carolina is the first Democratic primary or caucus state in which African American voters make up a sizeable share of voters, and Biden won thanks to overwhelming support from black voters.

Exit polls showed Biden winning 60 per cent of the black vote, well ahead of Sanders on 17 per cent and billionaire businessman Tom Steyer on 14 per cent.

Steyer, who spent over $US20 million ($31 million) on television advertising in South Carolina, announced he was dropping out of the race after the results came in.

Biden, Barack Obama's deputy for eight years, finished in a disastrous fourth and fifth place in Iowa and New Hampshire, leading many to doubt whether he could go the distance in the race to take on US President Donald Trump in November.

Just three days out from the crucial Super Tuesday contests - where a third of total nominating delegates will be awarded - Biden still faces significant hurdles in his bid to become the party's nominee.

His campaign is cash-poor and is being vastly out-spent on the airwaves by New York businessman Michael Bloomberg and Sanders.

Bloomberg announced on Saturday that he had taken out a major advertising buy to deliver a three-minute national address on the coronavirus.

Speaking to supporters in Houston after the results came in, Sanders said: "You cannot win 'em all. A lot of states out there, and tonight, we did not win in South Carolina. And that will not be the only defeat. There are a lot of states in this country. Nobody wins them all. I want to congratulate Joe Biden on his victory."

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Polls show Sanders ahead in the large and politically powerful states of California and Texas, which hold their primaries on Tuesday, thanks to a coalition of young, progressive and Hispanic voters.

The moderate vote is also likely to be split between Biden and several other candidates unless any other rivals drop out before Super Tuesday. Pete Buttigieg and Elizabeth Warren both said they would continue to fight on.

Democratic Party elites are likely to increasingly rally around Biden as the only viable alternative to Sanders, a self-declared democratic socialist they believe will drag down the Democratic vote in November.

Soon after the polls closed in South Carolina, former Virginia governor Terry McAuliffe endorsed Biden for the party's nomination.