“I know that the senator says she’s for Bernie. Well I’m for Barack. I think Obamacare worked,” Biden said, asking Warren and Sanders to explain how they would pay for their plans. “This is about candour, honesty, big ideas.”

Warren, a US senator from Massachusetts who has moved into second behind Biden in many opinion polls of the Democratic race, praised Obama’s health care efforts but said more was needed.

“Now the question is how best can we improve on it,” she said, adding that under Medicare for All, those at the top would pay more but the middle class would pay less.

Sanders, who sponsored the Medicare for All plan in the US Senate, said the programme based on the existing government-run Medicare programme for Americans 65 and older was the most cost-effective approach. Some analysts have estimated his plan would cost US$32 trillion over a decade.

Biden said his plan would give Americans more options, including staying with their plans if they like them.

“I’ve never actually met anybody who likes their health insurance company,” Warren fired back.

Candidates also took aim at Trump. Cory Booker called Trump a racist. Beto O’Rourke called him a white supremacist. And Kamala Harris said Trump’s hateful social media messages provided “the ammunition” for recent mass shootings.

Harris also attacked Trump on trade.

“The bottom line is this: Donald Trump, in office on trade policy, you know, he reminds me of that guy in The Wizard of Oz, you know, when you pull back the curtain, it’s a really small dude.”

Former US Housing Secretary Julian Castro accused Biden of flip-flopping in his description of his own plan.

“Are you forgetting what you said two minutes ago?” Castro, 44, asked Biden, 76, who is known for being prone to gaffes and has faced questions about his age.

When Mayor Pete Buttigieg of South Bend, Indiana, noted the exchange was what people did not like about politicians, Castro shot back: “That’s called an election.”

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US Senator Amy Klobuchar of Minnesota also tried to calm things down, noting: “A house divided cannot stand,” quoting from a famous speech by Abraham Lincoln.

The candidates made an effort to praise the legacy of Obama, after facing criticism from some Democrats after the last debate for attacking his policies on health care and immigration.

But Castro, who has lagged badly in opinion polls, accused Biden of being quick to tie himself to Obama when it suited him and walk away when it did not.

“He wants to take credit for Obama’s work but not have to answer any questions,” said Castro, who served in Obama’s administration.

The debate was narrowed to one night and 10 candidates after the party toughened the requirements for qualifying. The previous two Democratic debates in June and July were split over two nights.

Entrepreneur Andrew Yang opened the debate by promising to give US$1,000 a month to 10 Americans for the next year, calling it a “freedom dividend”. The offer drew laughs from a few other contenders.

“It’s original, I’ll give you that,” Buttigieg said.

The first two rounds of debates also were contentious, as Biden came under repeated attacks for his record on race and criminal justice reform and his views on health care.

But this time, the candidates tried to emphasise their areas of agreement. Biden praised O’Rourke for his work after a mass shooting in his hometown of El Paso, Texas, drawing a sustained ovation from the debate crowd.

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The sharp bickering during the first two rounds of the debates dismayed some Democrats, who have urged the candidates to rein in their attacks and focus on laying out their own affirmative agendas.

O’Rourke, who has called for gun licensing and a mandatory gun buy-back for assault weapons, was asked if he was going to take away people’s guns.

“I am if it was a weapon that was designed to kill people on a battlefield,” he said. “Hell yes, we’re going to take your AR-15, your AK-47.”