New York billionaire Mike Bloomberg was savaged by his Democratic rivals on his first debate on the presidential campaign trail.

Mr Bloomberg, the former New York mayor who was once a Republican, was forced to defend his record and past comments related to race, gender and his personal wealth in an occasionally rocky debate stage debut in Las Vegas.

The ninth debate of this cycle featured the most aggressive sustained period of infighting in the Democrats’ search for a presidential nominee.

The tension reflected growing anxiety among candidates and party leaders that the nomination fight could yield a candidate who will struggle to build a winning coalition in November to beat Donald Trump.

Election 2020 Debate
Democratic presidential candidates Mike Bloomberg, left, and Elizabeth Warren talk during a break at the Democratic presidential primary debate in Las Vegas (John Locher/AP)

Elizabeth Warren was in a fight for survival and stood out with repeated attacks on Mr Bloomberg.

She sought to undermine him with core Democratic voters who are uncomfortable with his vast wealth, his offensive remarks about policing of minorities and demeaning comments about women, including those who worked at his company.

Ms Warren labelled Mr Bloomberg “a billionaire who calls people fat broads and horse-faced lesbians”.

Meanwhile, Bernie Sanders lashed out at Mr Bloomberg’s policing policies as New York City mayor that he said targeted “African-American and Latinos in an outrageous way”.

And former vice president Joe Biden alleged that Mr Bloomberg’s “stop-and-frisk” policy ended up “throwing five million black men up against the wall”.

Election 2020 Trump
President Donald Trump monitored the debate while speaking at a campaign rally in Phoenix (Evan Vucci/AP)

Watching from afar, Mr Trump joined the Bloomberg pile on.

“I hear he’s getting pounded tonight, you know he’s in a debate,” Mr Trump said at a rally in Phoenix.

After the debate, Ms Warren told reporters: “I have no doubt that Michael Bloomberg is reaching in his pocket right now, and spending another hundred million dollars to try to erase every American’s memory about what happened on the debate stage.”

On a night that threatened to tarnish the shine of his carefully constructed image, Mr Bloomberg faltered when attacked on issues related to race and gender.

But he was firm and unapologetic about his wealth and how he has used it to effect change important to Democrats.

He took particular aim at Mr Sanders and his self-description as a democratic socialist.

Election 2020 Debate
Bernie Sanders is emerging as the frontrunner for the Democratic nomination (John Locher/AP)

“I don’t think there’s any chance of the senator beating Donald Trump,” Mr Bloomberg declared before noting Mr Sanders’ rising wealth.

“The best known socialist in the country happens to be a millionaire with three houses,” he said.

Mr Sanders defended owning multiple houses, noting he has one in Washington where he works, and two in Vermont, the state he represents in the Senate.

While Mr Bloomberg was the shiny new object on Wednesday, the debate also marked a major test for Mr Sanders, who is emerging as the frontrunner in the Democrats’ nomination fight, whether his party’s establishment likes it or not.

A growing group of donors, elected officials and political operatives fear that Mr Sanders’ uncompromising progressive politics could be a disaster in the general election against Mr Trump but they have struggled to coalesce behind a single moderate alternative.