Democratic frontrunner Bernie Sanders came under fierce attack from rivals in a chaotic debate on Tuesday, days ahead of a critical primary in South Carolina that could make or break former Vice President Joe Biden's campaign.

On an evening which featured bad-tempered bickering and frequent cross-talking – probably the last debate for several candidates – Michael Bloomberg was again a frequent punching bag for Elizabeth Warren while Biden, Pete Buttigieg and Amy Klobuchar focused their fire on Sanders.

Sanders defended his spending plans, his stance on guns and recent comments about Cuba, and insisted only he could inspire the voter turnout needed to beat President Donald Trump in November's election.

The candidates also strongly criticized Trump's response to the coronavirus, accusing him of cutting funding to the Centers for Disease Control and Prevention and stressing the seriousness of the situation.

Biden: I will win South Carolina

Biden made a forceful pitch to voters ahead of Saturday's South Carolina primary and expressed confidence, insisting "I will win" and making frequent appeals to black voters, including a pledge to nominate an African American woman to the Supreme Court. 

"I've worked like the devil to earn the vote of the African American community," he added. 

The Biden campaign announced after the debate that the 77-year-old had raised more money than on any day since he launched his candidacy, which has so far featured poor performances in Iowa and New Hampshire and a distant second place in Nevada. He is expected to be endorsed by influential South Carolina representative Jim Clyburn on Wednesday.

The former vice president laid into Sanders on his record on gun safety and Tom Steyer, the billionaire who is polling well in South Carolina, on his past investments in private prisons.

Sanders was booed by the audience when attempting to hit back at Biden, who had criticized him for voting to protect gun manufacturers from legal liability. "I have cast thousands of votes, including bad votes. That was a bad vote," Sanders conceded.

The frontrunner again defended his praise for Fidel Castro's literacy program in Cuba, arguing "do something good, you acknowledge that, but you don't have to trade love letters with them."

The self-proclaimed "democratic socialist" was also criticized over his spending plans, with Klobuchar accusing him of a 60 trillion U.S. dollar spending plan that amounted to "a bunch of broken promises that sound good on bumper stickers."

Warren broke with Sanders for one of the first times in the campaign, saying while she agreed with him on many issues, she would be the better president because she has a history of delivery.

"Bernie and I agree on a lot of things, but I think I would make a better president than Bernie," she said.

Buttigieg's down ballot warning

Buttigieg, the 38-year-old military veteran, drew sharp contrasts with Sanders and also raised the issue of down ballot elections in 2020.

He argued that nominating Sanders would mean "four more years of Donald Trump, Kevin McCarthy as Speaker of the House, and the inability to get the Senate in Democratic hands."

"The time has come to stop acting like the presidency is the only office that matters. Not only is this a way to get Donald Trump re-elected. We have a House to worry about. We have a Senate to worry about. And this is really important," Buttigieg, who may be reliant on super delegates at a brokered convention for a chance of the nomination, continued.

The former South Bend, Indiana, mayor performed well in the first three voting states but is lagging behind in South Carolina and like other moderates, has struggled for funding to compete on the airwaves on Super Tuesday. 

He added he was worried about an election between "Donald Trump with his nostalgia for the 1950s, and Bernie Sanders for his nostalgia for the revolutionary politics of the 1960s. This is about the future."

In a post-debate interview on CNN, Sanders said he beat Trump in 47 of the past 50 polls conducted. "The way you beat Trump is to have the largest voter turnout in this country," he added, insisting "we might want to listen to the people, the people voted for me in Iowa, New Hampshire and Nevada."

Warren-Bloomberg feud

The feud between Bloomberg and Warren continued from the previous debate in Nevada, with the Massachusetts senator hitting out at the former New York City mayor's past support for Republicans and his treatment of women in the workplace.

"I don't care how much money Mayor Bloomberg spends, the core of the Democratic Party will never trust him," Warren said. "He is the riskiest candidate on this stage."

She went on to raise the allegation than Bloomberg told a pregnant employee to "kill it," a claim he has denied. The senator attracted a huge spike in fundraising after her strong performance, dominated by attacks on Bloomberg, last week in Nevada. 

"For this senator, enough is never enough," Bloomberg, who ran campaign ads during commercial breaks in the debate and has already spent more than 400 million U.S. dollars, responded.