Sanders and Buttigieg come under fire ahead of New Hampshire vote

It came on the same day that US President Donald Trump sacked two of the highest profile witnesses in the impeachment probe

Former Vice President Joe Biden and Senator Bernie Sanders are viewed as the  leading Democratic contenders to take on US President Donald Trump in November's election. Courtesy: Reuters  

White House hopefuls Bernie Sanders and Pete Buttigieg - riding neck-and-neck in the polls ahead of the next Democratic primary contest - come under sustained attack on the debate stage from rivals seeking to challenge Donald Trump in November.

The veteran socialist was criticised for his left-wing politics while the young former mayor of South Bend, Indiana, aged 38, was derided for his inexperience.

The 78-year-old leftist Sanders, eyeing the moderate Buttigieg as his possible chief adversary, aimed his own shots at his far younger rival in the Manchester, New Hampshire debate - casting him as the candidate of Wall Street.

"I don't have 40 billionaires, Pete, contributing to my campaign," Mr Sanders said.

Mr Buttigieg and Mr Sanders finished atop the pack earlier this week in Iowa's chaotic caucuses, and both hope to renew the performance Tuesday in New Hampshire, as the Democratic Party seeks to pick a challenger to Mr Trump.

But Mr Sanders, a veteran senator calling for "political revolution," was in the firing line from several rivals, including former vice president and fellow septuagenarian Joe Biden who branded his policies too radical to unite Americans.

The 77-year-old Biden, fighting to keep his White House hopes alive after finishing an unnerving fourth in Iowa, insisted liberal policies like Sanders's flagship universal health care plan would be too divisive, expensive and difficult to get through Congress.

"How much is it going to cost?" Mr Biden asked about Sanders's Medicare for All bill which estimates the project would cost tens of trillions of dollars.

The attacks came on the same day that Mr Trump sacked two of his team after they testified as witnesses in his impeachment probe.

The sackings of Gordon Sondland, ambassador to the European Union, and Lieutenant Colonel Alexander Vindman, a former soldier who worked at the National Security Council, prompted accusations that the president is out for revenge.

The firings came two days after the Republican-majority Senate acquitted Mr Trump of charges that he abused his office and one day after he gave a victory speech branding his opponents as "evil."

Mr Sondland, a political appointee who got his post after donating $1 million to Mr Trump's inauguration, said in a brief statement, "I was advised today that the president intends to recall me effective immediately."

The ouster of Mr Vindman, a respected officer who was wounded in Iraq, was even more abrupt, when he was ordered out of his NSC offices at the White House.

He was "escorted out of the White House where he has dutifully served his country and his president," his lawyer David Pressman said in a statement.

"Vindman was asked to leave for telling the truth," Mr Pressman said.

Mr Vindman's twin brother Yevgeny, also a lieutenant colonel who worked as an attorney in the NSC, was fired simultaneously, US media reported.

Mr Trump has described the impeachment process as a hoax, denying there was anything wrong in his push for Ukraine to open a politically embarrassing investigation into Democratic presidential candidate Joe Biden's family.

On Friday, Mr Trump told reporters that he wants Republicans to retake control of the lower house of Congress in the next election and to "expunge" his impeachment.