Democratic White House contenders united in supporting the impeachment inquiry against Republican President Donald Trump at a debate in Atlanta last night that featured differences on policy details but few of the bitter attacks on one another that marked earlier encounters.

During the fifth debate in the Democratic race to pick a challenger to Mr Trump in the November 2020 election, the ten candidates aired differences on healthcare and taxing the wealthy, but kept the exchanges largely polite and instead heaped heavy criticism on Mr Trump.

US Senator Elizabeth Warren, who has pushed ambitious plans to tax wealth and create a government-run healthcare plan, and Pete Buttigieg, the 37-year-old mayor of South Bend, Indiana, who has been rising in the polls, escaped sustained criticism from their rivals.

Mr Buttigieg, running to be the first openly gay president, was pressed on his failure to make inroads with African-Americans - a key Democratic constituency - and drew a parallel to his experience being gay.

"I do not have the experience of ever being discriminated against because of the colour of my skin," Mr Buttigieg said.

"I do have the experience of feeling like a stranger in my own country, turning on the news and seeing my own rights coming up for debate."

He also defended his relative lack of experience, saying it was not traditional establishment Washington experience but "the right experience to take on Donald Trump".

US Senator Amy Klobuchar, who had questioned if a woman with Mr Buttigieg's experience would make the debate stage, said he deserved his standing but there was a double standard when it came to women candidates.

"Otherwise we could play a game called 'Name your favourite woman president,'" Ms Klobuchar said. No woman has served as US president.

She added: "If you think a woman can't beat Trump, (House Speaker) Nancy Pelosi does it every day."

Democratic presidential hopefuls Pete Buttigieg (L), Elizabeth Warren and Joe Biden

Hours after the fourth day of public impeachment hearings in Congress, the candidates repeatedly blasted Mr Trump and said the president's efforts to press Ukraine to investigate former Vice-president Joe Biden, a leading Democratic presidential contender, were an example of the administration's corruption.

The Democratic-led House of Representatives has launched an impeachment inquiry into Trump's bid to get Ukraine to investigate Mr Biden and his son Hunter, who served on the board of Ukrainian gas company Burisma.

The candidates said Mr Trump's actions had forced lawmakers to hold him accountable.

"We have to establish the principle that no one is above the law, we have a constitutional responsibility and we need to meet it," Ms Warren said.

An early supporter of Trump's impeachment, she said she would try to persuade Senate Republicans that the president should be removed.

Ms Warren, who is campaigning on the platform of getting money out of politics, also said that no donor would be appointed ambassador under her presidency.

"We are not going to give away ambassador posts to the highest bidder," she said.

Asked if he would support a criminal investigation into Mr Trump after he leaves the White House, Mr Biden said he would leave it to the Department of Justice to decide whether Mr Trump should be prosecuted for his actions.

"If that was the judgement, that he violated the law, and he should be in fact criminally prosecuted, then so be it. But I would not direct it, and I don't think it’s a good idea that we mock, that we model ourselves after Trump and say: 'Lock him up,’" he said.

Candidates Bernie Sanders and Kamala Harris

Mr Biden said he had learned something important from the impeachment inquiry.

"I learned, number one, that Donald Trump doesn't want me to be the nominee. That's pretty clear," he said.

The Democratic debate comes just 11 weeks before the first nominating contest in Iowa, on 3 February, raising the stakes for the participants as they tried to make an impression with voters before time runs out.

The candidates condemned Mr Trump's conduct of foreign policy beyond his dealings with Ukraine.

US Senator Kamala Harris said Mr Trump had been "punked" in his dealings with North Korea and "traded a photo-op for nothing."

Mr Biden said Mr Trump, who has met three times with North Korean leader Kim Jong-un, gave Pyongyang "everything they wanted".

US Senator Bernie Sanders criticised Mr Trump for conducting foreign policy "in a tweet at 3 o'clock in the morning".

The Democratic White House race has featured a three-way battle at the top of recent national polls between moderate Mr Biden and progressive leaders Ms Warren and Mr Sanders.

But Mr Buttigieg has taken the lead in two recent polls in Iowa and is on the move in New Hampshire, another early voting state.