After a projected fifth-place finish in South Carolina, Elizabeth Warren closes out a fourth consecutive primary with a loss in her race for the Democratic presidential nomination.

"I'll be the first to say that the first four contests haven't gone exactly as I'd hoped", she told supporters in Houston, Texas. "[But] Super Tuesday is three days away."

Joe Biden's crucial victory in South Carolina gives the former vice president a much-needed boost after disappointing results in the first three primary states after his once-ascendent campaign struggled to capture support.

Bernie Sanders, who made history as the first candidate to win popular votes in the first three primaries, is projected to finish in second place in South Carolina, edging closer to 20 per cent to Biden's 50 per cent. Warren, meanwhile, is expected to come in fifth - trailing behind her third place finish in Iowa and fourth place finishes in New Hampshire and Nevada.

The Massachusetts senator congratulated Biden on his win but made it clear that she has no intention of dropping out of the race, banking on winning contests in 14 states - including Texas - on Super Tuesday, when more than a third of all delegates will be selected to choose the party's nominee.

"We want to gain as many delegates to the convention as we can", she said. "It might take days or even longer to know the results from Super Tuesday, but they will be critical in sorting out who will be the nominee this year. My campaign is built for the long haul."

She took aim at Donald Trump's response to a coronavirus outbreak, after slamming Mike Pence - who was tapped to lead the administration's response - as "the worst person" to be appointed to the role, and being the first among the Democratic candidates to announce a plan to combat infectious diseases.

With fears of a pandemic and ensuing economic crisis, "who do you trust to actually run this country?", she asked her supporters in Texas.