Leading Democratic White House contenders Joe Biden and Elizabeth Warren clashed on their healthcare plans during the opening minutes of a debate on Thursday, with Biden quickly criticizing Warren’s call for a Medicare for All approach, reports Trend referring to Reuters.

Biden, the former vice president under Barack Obama, said he would build on Obama’s landmark healthcare bill and accused Warren and her fellow progressive Bernie Sanders of wanting to tear it down.

"I know that the senator says she’s for Bernie. Well I’m for Barack. I think Obamacare worked,” Biden said. "This is about candor, honesty, big ideas.”

Warren, who has moved into second behind Biden in opinion polls of the Democratic race to face Republican President Donald Trump in the November 2020 election, praised Obama’s healthcare efforts, but said: "Now the question is how best can we improve on it,” adding Medicare for All was the best approach.

Sanders, who sponsored the Medicare for All plan in the U.S. Senate, weighed in to support Warren, saying it was the most cost-effective approach. Medicare for All would provide federal health insurance coverage for all Americans based on the existing government-run Medicare program for Americans 65 and older

The battle over healthcare has been the biggest issue of disagreement in the Democratic race. While all Democrats support universal healthcare, they disagree on how to get there, with Biden and others calling for a government-run healthcare plan being one option alongside private insurance.

Entrepreneur Andrew Yang opened the debate by promising to give $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,” said Mayor Pete Buttigieg of South Bend, Indiana.

The top 10 candidates for the party’s presidential nomination are sharing a stage for the first time. The previous two Democratic debates in June and July were split over two nights.

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

Warren and Sanders, old allies who have promised not to criticize each other, teamed up during the second round of debates in Detroit to defend their progressive agenda when attacked by more moderate candidates.

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.