Democratic presidential hopeful Massachusetts Senator Elizabeth Warren speaks to the press in the spin room during the fourth Democratic primary debate of the 2020 presidential campaign season co-hosted by The New York Times and CNN at Otterbein University in Westerville, Ohio on Tuesday. Photo: VCG

Surging White House hopeful Elizabeth Warren faced a barrage of attacks from fellow Democrats at the party's fourth 2020 debate Tuesday, cementing her status as a front-runner in the race to challenge Donald Trump.

The president himself loomed large as the dozen Democratic contenders trained their fire on him, calling for his impeachment and assailing a Syria troop pullout that Joe Biden slammed as "shameful."

"The impeachment must go forward," thundered Warren, the progressive senator who is neck and neck with former vice president Biden at the head of the 2020 nomination race - a stance loudly echoed by her fellow Democrats on stage.

Washington's impeachment brawl has dominated US politics for weeks, centered on Trump's effort to press Ukraine to dig up dirt on Biden.

Dragged into the scandal, Biden made clear he believes Trump should be impeached - and pushed back hard on the president's charge he himself intervened in Ukraine to protect his son Hunter.

"My son did nothing wrong. I did nothing wrong," Biden said, when asked about his son's employment with a Ukrainian company, which even some Democrats say held the appearance of a conflict of interest. 

The Ukraine scandal thrust Biden into the spotlight, while 78-year-old liberal Senator Bernie Sanders - the other heavyweight in the race - was under pressure to project fortitude two weeks after being sidelined by a mild heart attack.

But it was Warren who weathered the sharpest attacks from her fellow Democrats - on everything from kitchen-sink issues such as healthcare and taxation, to foreign policy and even business automation.

On health - a flashpoint issue for American voters - Warren faced stiff blowback from moderates who challenged her to come clean on how much her "Medicare for All" plan would cost.

"The difference between a plan and a pipe dream is something that you can actually get done," said Senator Amy Klobuchar, while Biden called Warren's health policy "vague."

Ex-congressman Beto O'Rourke accused Warren - who advocates a wealth tax on the biggest fortunes - of "punitive" policies, and "pitting some part of the country against the other."