HOUSTON (REUTERS) - Democratic presidential front runner Joe Biden clashed with progressive challengers Elizabeth Warren and Bernie Sanders on healthcare in a debate on Thursday (Sept 12), defending Obamacare and accusing them of not being honest about the cost of their plans.

Mr Biden, who served as vice-president for eight years under former president Barack Obama, said he would build on Mr Obama's landmark healthcare law and accused Ms Warren and Mr Sanders of wanting to tear it down with their support for Medicare for All, a government-run healthcare plan that would eliminate private insurance.

"I know that the senator says she's for Bernie. Well I'm for Barack. I think Obamacare worked," Mr Biden said.

"This is about candor, honesty, big ideas."

Ms Warren, a US senator from Massachusetts, who has moved into second place behind Mr Biden in opinion polls of the Democratic race to face Republican President Donald Trump in the November 2020 election, praised Mr Obama's healthcare efforts.

"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.

Mr Sanders, who sponsored the Medicare for All plan in the US Senate, said the plan based on the existing government-run Medicare programme for Americans aged 65 and older was the most cost-effective approach. Some analysts have estimated his plan would cost US$32 trillion (S$44 trillion) over a decade.

The battle over healthcare has been the biggest area of disagreement in the Democratic race, with candidates clashing on how to improve on Obamacare.

Mr 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," Ms Warren responded.

Former housing secretary Julian Castro accused Mr Biden of flip-flopping in his description of his own plan.

"Are you forgetting what you said two minutes ago?" Mr Castro asked Mr 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, Mr Castro shot back: "That's called an election."

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

The debate featured the top 10 candidates for the party's presidential nomination sharing a stage for the first time. 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," Mr Buttigieg said.

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

But several of the candidates tried to emphasise their areas of agreement and turn their criticism towards Mr Trump.

Mr Biden praised former representative Beto O'Rourke for his work after a mass shooting in his hometown of El Paso, Texas, drawing a sustained ovation from the debate crowd.

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