Democratic frontrunner Joe Biden went on the offensive on Wednesday against his main 2020 opponents, but was rapidly assailed on the debate stage over key issues like health care, race, immigration and criminal justice.

Tensions rose rapidly between the former vice president and Senator Kamala Harris, the most prominent African American in the field, as the two reprised their clash from a month earlier at the debut debate.

But while Biden aimed aggressive attacks at Harris and her health care plan, other rivals in the second night of the two-night, 20-candidate debate sought to undercut him on a host of issues.

Biden is leading polls for the nod to take on President Donald Trump in 2020, and nearly all of the other nine Democrats on stage attacked him at some point.

He found himself in a series of sharp exchanges on the central campaign issue of health care along with his stance on climate change and his past legislative record, in particular his failure to take decisive action against illegal immigration.

When Biden jousted with Harris about her "double talk" on her own modified Medicare for All plan, which he noted would take 10 years to kick in, Harris shot back: "You're simply inaccurate in what you're describing."

She said Biden's proposal for government-backed health insurance for those who want it "leaves out almost 10 million Americans" from coverage.

Biden's advisors had urged him to be more aggressive after his lackluster first debate performance, but the 76-year-old struck a curious opening note on Wednesday.

As the debate began, Biden greeted Harris, a 54-year-old former California attorney general, by shaking her hand and saying, "Go easy on me, kid."

She did not.

Harris renewed the criticism that gave her a viral moment in the first debate by accusing Biden of making light of his work with segregationists in the Senate in the 1970s.

"The vice president has still failed to acknowledge that it was wrong to take the position that he took at that time," Harris said.

Rare unity

The squabbling came after deep fault lines between the party's centrist and progressive wings were exposed on the first debate night, which featured leading liberals Bernie Sanders and Elizabeth Warren.

In one of the few moments of broad unity onstage Wednesday, several candidates joined Biden in condemning Trump.

Congresswoman Tulsi Gabbard said the president "is not behaving like a patriot," while Julian Castro, the only Latino in the field, branded Trump an outright "racist".

But much of the night was consumed with clashes between candidates, and progressives were quick to highlight the ideological divide with moderates on multiple fronts.

"Middle ground approaches are not enough," said Washington Governor Jay Inslee, who has released a trillion-dollar plan to tackle climate change. "We must confront the fossil fuel industry."

New York Mayor Bill de Blasio, in the race's left lane, laid down the gauntlet by saying he was fully prepared to "restructure" the US economy and society.

"We will tax the hell out of the wealthy," he boomed.

Low-polling candidates like de Blasio, Inslee, Senator Kirsten Gillibrand and entrepreneur Andrew Yang were desperate for standout moments to boost their exposure and justify staying in the race. Few were evident.

The stakes are high, as tougher entry requirements for the next debate in September are expected to winnow down the field of 20 candidates by as much as half.

'Phony rhetoric'

Booker took a stab at a breakout by entering into a searing exchange with Biden over his criminal justice record.

"There are people right now in prison for life for drug offenses, because you stood up and used that tough-on-crime phony rhetoric that got a lot of people elected, but destroyed communities like mine," said Booker, who is black.

Biden countered by arguing that Newark's police department was troubled when Booker was mayor there and advocated a stop-and-frisk policy targeting blacks.

Biden also faced backlash for his positions on gender equity and abortion rights, and for his 2002 vote in favour of going to war in Iraq after 9/11.

"I did make a bad judgment," Biden admitted, referring to his trusting of President George W. Bush's justification for the invasion.

Castro attacked Biden for standing with Obama's ramped-up deportation policy while vice president, and for refusing to support Castro's controversial proposal to decriminalize unauthorized border crossings as a candidate.

Biden has often linked himself closely to Obama, in part to attract support from the black community.

But when Biden refused to say whether he would stand by the beloved Democratic president or align with today's left-leaning party, Booker twisted the knife.

"You invoke President Obama more than anybody in this campaign," Booker said. "You can't do it when it's convenient and then dodge it when it's not."