WESTERVILLE, OHIO (BLOOMBERG) - Ms Elizabeth Warren's rivals targeted her new status as a front runner in the Democratic race in a televised debate on Tuesday (Oct 15), questioning and criticising her for not explaining the cost of her healthcare plan and for proposing dramatically higher taxes on the ultra-wealthy.

The other front runner, Mr Joe Biden, had to confront questions about his son's work in Ukraine. But he avoided attacks from the other 11 candidates on the stage, who all were united in decrying President Donald Trump's conduct in office and on the need for him to be impeached.

"Look, my son did nothing wrong. I did nothing wrong. I carried out the policy of the United States government in rooting out corruption in Ukraine," Mr Biden said from the stage of Otterbein University in Westerville, Ohio.

"What I think is important is we focus on why it's so important to remove this man from office."

From the outset, the fourth Democratic debate was a departure from the three earlier contests, which started with policy arguments over health insurance and immigration.

But it shifted quickly back to healthcare, giving an opening to Mr Pete Buttigieg and others to challenge Ms Warren over her refusal to say that the Medicare for All plan she supports would raise taxes on the middle class, even if their net health costs would go down.

What matters is "what kinds of costs middle-class families are going to face", Ms Warren, a Massachusetts senator, said.

But opponents who favour keeping the Affordable Care Act in place and adding to it accused Ms Warren of evasiveness.

Mr Biden said it was "awfully important" to be "straightforward". Mr Buttigieg said Ms Warren was asked "a yes or no question that did not get a yes or no answer".

"Your signature, senator, is to have a plan for everything, except for this," Mr Buttigieg said.

HIGHER TAXES

Ms Warren has backed Mr Bernie Sanders' Medicare for All proposal rather than offering her own plan. Mr Sanders readily conceded that his plan would come with a tax increase for the middle class, but said that it would be balanced by the elimination of premiums and other out-of-pocket healthcare costs.

"Taxes will go up. They're going to go up," Mr Sanders said.

Both Ms Warren and Mr Sanders came under criticism over their proposals to dramatically raise taxes on the very wealthy.

Ms Warren accused her fellow Democratic presidential candidates of putting billionaires ahead of schoolchildren because many of them were slow to endorse a wealth tax.

Her endorsement of a 2 per cent tax on Americans' wealth that tops US$50 million (S$68.5 million) drew a sharp contrast between progressives like her and Mr Sanders, and the rest of the Democratic field.

But others bristled at her suggestion and said it was unworkable and divisive.

WEALTH TAX

"I think Senator Warren is more focused on being punitive or pitting one part of the country against the other," said former Representative Beto O'Rourke.

"I want to give a reality check to Elizabeth. No one on this stage wants to protect billionaires," said Senator Amy Klobuchar.

Referring to Mr Tom Steyer, the billionaire investor making his first appearance on the debate stage, she said: "Not even the billionaire wants to protect billionaires."

Ms Warren said she didn't have a beef with billionaires - she just wants them to pay a bigger share for federal investments in education and infrastructure that helped make them rich.

Her plan would earmark that money for early childhood education, free college and student loan relief.

The attacks on Ms Warren had a hidden benefit for her. Halfway through the debate, she had the most speaking time of anyone on stage at roughly 12 minutes, more than four minutes ahead of the next-highest, Mr Biden, according to a tracker by the New York Times.

Mr Buttigieg and Mr O'Rourke, who both have struggled to find a way to break into the top tier of candidates, clashed over Mr O'Rourke's proposal for mandatory buybacks of military-style semi-automatic rifles.

"With 16 million AR-15s and AK-47s out there, they're also too dangerous to own. Every single one of them is a potential instrument of terror," Mr O'Rourke said.

But he was vague on how such a law would be enforced, saying only: "I expect my fellow Americans to follow the law."

"Congressman, you just make it clear that you do not know how this is going to take weapons off the street," Mr Buttigieg said.

He said Mr O'Rourke's proposal was getting in the way of passing universal background checks and "red flag" laws.

The three presidential candidates in their 70s tried to reassure Democrats that they could handle the physical rigours of holding the Oval Office.

Mr Sanders, 78, appeared at Tuesday's debate two weeks after suffering a heart attack and undergoing emergency surgery to install stents in his coronary arteries.

He said he would demonstrate his fitness by maintaining his schedule of mass rallies.

"We're going to be mounting a vigorous campaign all over this country. That's how I'm going to reassure the American people," he said.

Mr Biden, 76, promised to release his medical records before the Iowa caucuses on Feb 3 and bristled at a suggestion he should do so sooner.

"Before Iowa, I'm the only guy that's released anything up here," referring to his earlier release of 21 years of tax returns.

And Ms Warren, 70, promised to "out-work, out-organise and out-last anyone - and that includes President Trump, Vice-President Pence or whoever else the Republicans get stuck with".

Democrats were solidly unified on one issue: impeaching Mr Trump.

"Sometimes there are issues that are bigger than politics and I think that is the case," Ms Warren said.

"This man will not be able to break the law over and over without consequences" and moving ahead on impeachment is "about the next president and the next president and the next president" and the future of the country.

The impeachment question - rooted in part on Mr Trump's efforts to investigate the business dealings of Mr Joe Biden's son, Hunter - underscored how much the backdrop of the election has changed since House Speaker Nancy Pelosi launched the inquiry into Mr Trump last month.

Three House committees are investigating whether the President withheld aid to Ukraine as leverage to get the country's government to investigate Mr Hunter Biden.

Mr Hunter Biden, 49, acknowledged making a mistake in working for Burisma Holdings earlier on Tuesday. Mr Joe Biden said he had nothing more to add and none of his rivals used the opportunity to criticise him.

"My son's statement speaks for itself. I did my job. I never discussed a single thing with my son when it came to Ukraine," Mr Joe Biden said.

The candidates were nearly unified in decrying Mr Trump's withdrawal of US troops from Syria, which created a vacuum that Turkey has quickly filled by attacking US-allied Kurdish forces.

Mr Biden cited his experience as vice-president and past meetings with the leaders of Russia and Turkey to argue that he is best suited to undo the work of an "erratic, crazy president who knows not a thing about foreign policy and operates out of fear for his re-election".

Mr Buttigieg, a veteran with service in Afghanistan, said Mr Trump "is undermining the honour of our soldiers" and "you might as well go after their body armour next".

The debate could be the last gasp for some of the 12 candidates on stage - the largest line-up ever.

Only Mr Biden, Ms Warren and Mr Sanders register double digits in polls.