Frontrunner Joe Biden went on the offensive 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.

In a high-octane clash, 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 plan, which maintains and builds on the Affordable Care Act known as Obamacare.

Warren, a rising star in the race, and Sanders, a fixture from the 2016 campaign who launched a liberal political revolution, each put up a spirited defense.

"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 promised "those at the very top" would bear the cost.

Sanders, who advocates a shift away from private health insurance, vowed to "finally make sure that every American has health care as a human right, not a privilege."

Their three-way battle kicked off a marathon debate, as the rival Democrats highlighted their differences on immigration, trade tariffs, criminal justice reform and the wars in Iraq and Afghanistan, and lower-polling candidates scrambled for breakout moments.

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

"There's enormous, enormous opportunities - once we get rid of Donald Trump," Biden said during his opening remarks.

Addressing a Thursday dinner with Republican lawmakers, Trump's gloves came off too as he reeled off his favorite 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.

Dominant top three

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, concerns he and his team have roundly dismissed.

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

But Biden steered clear of any embarrassing blunders in Houston, and parried attacks from the likes of Sanders, who accused Biden of the "big mistake" of voting to authorize the war in Iraq.

Biden maintains a grip on pole position with 26.8 percent support, despite a noticeable dip in recent weeks, according to a poll average compiled by RealClearPolitics.

Sanders, at 78 the oldest candidate in the field, is on 17.3% support, narrowly ahead of the 70-year-old Warren at 16.8.

Biden enjoys strong support in particular from African-American communities and from working-class whites who appreciate his blue-collar appeal and believe he is best able to beat Trump, a top priority for Democratic voters.

While Warren's stock has risen, the campaigns of others such as Senator Kamala Harris and 37-year-old South Bend Mayor Pete Buttigieg have stalled.

For the second tier, including ex-congressman Beto O'Rourke, Senator Cory Booker, Senator Amy Klobuchar and Castro - all polling under 3% -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 for reining in gun violence.

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

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

Navigating between the leaders and the strugglers is tech entrepreneur Andrew Yang, who has made establishing a universal basic income to offset job losses a key part of his campaign.

He grabbed the spotlight at the debate's kickoff by promising to give a "freedom dividend" of $1,000 per month to 10 American families for a year.