Former US Vice President Joe Biden scored a convincing victory in South Carolina's Democratic primary on Saturday, riding a wave of African American support and ending the winning streak of Vermont senator and rival Bernie Sanders.

The victory came at a crucial moment in Biden's 2020 bid as the moderate Democrat bounced back from underwhelming performances in the first three contests.

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The race now quickly shifts to next week's "Super Tuesday," when voters in 14 states award one-third of the total number of presidential delegates.

"We are very much alive," Biden declared at an exuberant post-election rally. "For all of you who have been knocked down, counted out, left behind - this is your campaign."

The South Carolina primary was the first major test of the candidates' appeal among black voters. And while it gave the 77-year-old Biden a win when he most needed it, he must still prove that he has the financial and organisational resources to dramatically expand his campaign in the next 72 hours.

Seven candidates remain in the race to take on incumbent Donald Trump in the November election. As well as Biden and the 78-year-old Sanders, who came second, other contenders include Elizabeth Warren, a liberal senator from Massachusetts, and billionaire former New York Mayor Michael Bloomberg.

The Associated Press declared Biden the winner just after the polls closed in South Carolina, basing the call on data from AP VoteCast, a survey of the electorate conducted for the AP by NORC at the University of Chicago. The survey showed a convincing win for Biden.

Sanders congratulated Biden on his first win and said it was nothing for his supporters to worry about.

"Tonight, we did not win in South Carolina. That will not be the only defeat. A lot of states in this country. Nobody wins them all," he told a cheering crowd in Virginia, one of 14 states to vote next week. "Now, we enter Super Tuesday."

Sanders is spending the lead-up to Super Tuesday campaigning in Boston, Warren's political turf, and Minnesota, the home state of Amy Klobuchar, who is looking for her first win.

'Springboard'

Elected officials inclined to embrace Biden's moderate politics had been reluctant to support him after poor finishes in Iowa and New Hampshire and a distant second place in Nevada last week. But fearing Sanders's potentially polarising progressive policies, they are searching for an alternative who can defeat Republican incumbent Donald Trump in November.

Senior Biden adviser Symone Sanders shifted away from calling South Carolina Biden's "firewall" and instead called it a "springboard," on par with how the state boosted the presidential aspirations of Barack Obama in 2008 and Clinton in 2016.

South Carolina was the first major test of the candidates' popularity with African American voters, who will be critical in the rest of the primary season and in the election itself.

Biden won 60 percent of the votes cast by African Americans. He also did well with older voters, women, moderates and conservatives and regular churchgoers, according to AP VoteCast.

Sanders earned the support of roughly 15 percent of African American voters, while billionaire businessman Tom Steyer won 16 percent.

There was also evidence that Biden's status as former President Barack Obama's two-term vice president helped him win over African Americans.