Former New York mayor Michael Bloomberg endured a punishing assault in his debut Democratic presidential debate on Wednesday from rivals who savaged him for parachuting in late and throwing astronomical amounts of money at the race.

The Las Vegas showdown came at a tense time in the party's nomination race, with Bernie Sanders coalescing as the front-runner and some challengers fighting for survival.

US media magnate Bloomberg found himself targeted from the start, as candidates from Sanders to former vice president Joe Biden impugned the man whose sudden prominence in polling has scrambled the race to defeat President Donald Trump.

“Understand this: Democrats take a huge risk if we just substitute one arrogant billionaire for another,” Senator Elizabeth Warren said in a fiery attack on Bloomberg.

“Democrats are not going to win if we have a nominee who has a history of hiding his tax returns, of harassing women, and of supporting racist policies,” she added.

Sanders hit Bloomberg hard on his vast wealth at a time of “grotesque” income inequality in America.

“Mike Bloomberg owns more wealth than the bottom 125 million Americans,” Sanders said in one of the night's many abrasive moments. “That's immoral.”

All eyes were on Bloomberg's first appearance on the debate stage after spending hundreds of millions of dollars on campaign advertising.

He endured a very public vetting and struggled to highlight his roles as problem solver, businessman, city manager and philanthropist.

He found himself on the defensive when pressed to explain sexual harassment claims against him and employees, his delay in releasing his taxes and more.

Political analyst Aaron Kall of the University of Michigan said Bloomberg proved “really weak” in the face of the onslaught.

“It'll be interesting to see [...] whether that's disqualifying for a lot of people that have recently gone on his side,” Kall said.

Bloomberg at one point managed to push back forcefully against Sanders, saying if the self-declared democratic socialist is the nominee, “we will have Donald Trump for another four years”.

But Trump himself was dismissive of Bloomberg's performance onstage.

“He was stumbling, bumbling and grossly incompetent. If this doesn't knock him out of the race, nothing will,” the president tweeted.

Sanders in front

Sanders has been buoyed by a strong showing in Iowa, a New Hampshire victory and a surge in polling, with the next nominating contest in Nevada just three days away.

But establishment Democrats have begun public hand wringing about the prospects of Sanders taking the reins of a party seeking to make Trump a one term president.

Pete Buttigieg, the moderate young former mayor of South Bend, Indiana who scored a surprise narrow victory in Iowa, levelled a hit on both Sanders and Bloomberg with a withering critique.

“We shouldn't have to choose between one candidate who wants to burn this party down and another candidate who wants to buy this party out,” Buttigieg said.

A Washington Post-ABC News poll out Wednesday showed Sanders with a commanding double digit lead nationally, at 32 per cent.

Sagging former front-runner Biden was second at 16 pc, followed by Bloomberg at 14 and Warren at 12.

Biden's poor showing in the first two states placed him under enormous pressure to do well in Nevada and then South Carolina, which votes on February 29.

He spoke evocatively on climate change, and criticised Sanders over immigration policy and gun reform, but it remained to be seen whether it will help him in Nevada and beyond.

Super Tuesday strategy

While Sanders and other White House hopefuls have spent months barnstorming early states, billionaire Bloomberg jumped late into the Democratic contest.

He is going all in on so-called Super Tuesday on March 3, when 14 states including California and Texas vote on choosing a Democratic nominee.

“I have no doubt that Michael Bloomberg is reaching in his pocket right now and spending another $100 million to try to erase every American's memory about what happened on the debate stage,” Warren told reporters on Wednesday.

While Sanders leads, Bloomberg is surging on the national stage. Two separate polls released on Tuesday show him leapfrogging rivals to claim second spot behind Sanders, with Biden third.

For Warren and fellow candidate Senator Amy Klobuchar, the debate was a critical chance to convince voters that they belong in the race as it goes national.

As Democrats trained fire on one another, Klobuchar reminded candidates that their real opponent was the White House occupant.

“We have not been talking enough about Donald Trump,” she said.