Sanders and Warren defend progressive policies in US Democratic debate

Some contenders argued the proposal pushed by the progressives would take private insurance away from people who currently prefer their coverage and force a tax increase on the middle class.

"Why do we have to be the party of taking something away from people?" said Delaney, who has been critical of Medicare for All.

He argued that Democrats should not back proposals that would eliminate private insurance.

"They are running on telling half the country that your health insurance is illegal," Delaney said, pointing to Warren and Sanders.

Warren interjected to defend the proposal.

"We're not trying to take healthcare away from anyone.

That's what Republicans are trying to do."

Sanders, who has introduced a Medicare for All bill in the U.S. Senate, said: "Healthcare is a human right, not a privilege. I believe that, I will fight for that."

The candidates also differed on immigration policy - disagreeing on whether illegal border crossings should be decriminalized.

"You don't have to decriminalize everything," Bullock said.

"What you have to do is to have a president in there with the judgment and the decency to treat someone that comes to the border like one of our own."

Warren disagreed, saying: "We need to expand legal immigration, we need to create a path for citizenship not just for dreamers but for grandmas and for people who have been working here in the farms and for students who have overstayed their visas."

"You are playing into Donald Trump's hands," Bullock said.

Lower-tier candidates were aiming to produce the kind of post-debate surge that US Senator Kamala Harris enjoyed after a strong performance in the first debate after confronting Biden on the issue of race.

The two nights in Detroit could be the last chance for many contenders to debate on the national stage, as the Democratic National Committee will double its fundraising and polling requirements to qualify for participation in the next debates in September and October.

Ten more candidates will face off on Wednesday night, including Biden and Harris.