Frontrunner Joe Biden went on the offensive on Thursday in the third Democratic debate of the 2020 White House race, clashing with top rivals on the fraught issue of health care in America and brushing off attacks from lesser challengers.

Under pressure to appear in command — and dispel doubts over his stamina — the 76-year-old Biden pushed hard against liberals Bernie Sanders and Elizabeth Warren in an almost three-hour showdown in Houston, Texas.

While the 10 Democrats seeking the party nomination found common ground in their determination to oust Donald Trump, and on the urgency of tackling climate change, their differences were on stark display when it came to health care reform — a stated priority for them all.

It was the top-polling trio that shared the limelight, with Biden snatching the most speaking time of all, as lesser candidates fought for oxygen on stage.

Throughout the campaign Democrats have split between advocates of revolutionary upheaval and incremental change, and Thursday's debate was no different. In a high-octane clash, the centrist Biden accused his fellow septuagenarians, senators Sanders and Warren, of pushing pipe dreams without a plan to fund them.

"I lay out how I can pay for it, how I can get it done, and why it's better," the former vice president said of his health plan, which builds on the Affordable Care Act known as Obamacare.

Warren, a rising star in the race, and Sanders, a liberal fixture from the 2016 campaign, each put up a spirited defence.

"I know what's broken, I know how to fix it and I'm going to lead the fight to get it done," promised Warren, who has electrified town halls and impressed voters with her exhaustive policy platforms.

On health care reform she vowed "those at the very top" would bear the cost, while Sanders pledged to make sure "every American has health care as a human right". Their three-way battle kicked off a verbal marathon, as the rival Democrats highlighted differences on immigration, trade tariffs and the wars in Iraq and Afghanistan.

But they stood united on one key point: ousting Trump — who several candidates attacked as racist — from the White House.

"There's enormous, enormous opportunities — once we get rid of Donald Trump," Biden said.

No big blunder

Addressing a dinner with Republican lawmakers, Trump's gloves came off too as he reeled off his favourite insults against Sanders, Warren and Biden — "Crazy Bernie," "Pocahontas" and "Sleepy Joe".

"Our country will go to hell if any of these people get in," Trump warned.

All eyes were on Biden for debate number three of what looks to be a bruising election cycle, after a summer of verbal miscues raised doubts about his age and mental clarity.

Seeking a breakout moment, low-polling candidate Julian Castro dared challenge Biden on the sensitive issue, accusing him of "forgetting what you said just two minutes ago" — drawing boos for the below-the-belt attack.

But Biden avoided embarrassing blunders in Houston and parried attacks from the likes of Sanders, who accused him of the "big mistake" of voting for the war in Iraq.

Biden maintains a grip on pole position with 26.8 per cent support, despite a recent dip, according to a poll average compiled by RealClearPolitics.

Sanders, 78, is on 17.3 per cent support, narrowly ahead of the 70-year-old Warren at 16.8.

Biden enjoys strong backing from African-American communities and from working-class whites who appreciate his blue-collar appeal and believe he is best able to beat Trump.

Warren: quiet but effective

While Biden was widely seen as having surpassed expectations, there were solid performances from Senator Kamala Harris, ex-congressman Beto O'Rourke and perhaps crucially Warren — who true to form reeled off numerous policy details intertwined with personal anecdotes.

"It was Elizabeth Warren who quietly turned in the best performance," Princeton University history professor Julian Zelizer wrote on CNN.

For the second tier, including South Bend Mayor Pete Buttigieg, Senator Cory Booker, Senator Amy Klobuchar and Castro, a breakout is critical to stay relevant.

O'Rourke, a native of El Paso which suffered a mass shooting in August, was praised by rivals for his eloquent calls to rein in gun violence.

In turn he dialed up the rhetoric in calling for a mandatory buyback of military-style assault weapons.

"Hell, yes, we're going to take your AR-15, your AK-47," he boomed.