Former New York mayor Michael Bloomberg will confront rival presidential hopefuls for the first time on the Democratic debate stage on Wednesday.
Mr Bloomberg will face barbs from competitors irked by his rising prominence in the race to take on US President Donald Trump.
But Bernie Sanders will take to the stage as clear frontrunner, buoyed by wins in Iowa and New Hampshire, and surging in the polls with the next nominating contest four days away in Nevada.
A Washington Post-ABC News poll on Wednesday showed Mr Sanders with a commanding lead nationally, at 32 per cent.
Former frontrunner Joe Biden was second at 16 per cent, followed by Mr Bloomberg at 14 and Elizabeth Warren at 12.
While Mr Sanders and the other White House hopefuls spent months campaigning in early states, billionaire Mr Bloomberg came late into the Democratic nomination contest.
His rise in the polls, fuelled by huge spending on campaign advertising, has helped him to qualify for his first 2020 debate. His performance will be a closely watched public test.
It also has boosted the chances of the November election being a very unique show of one white male New York billionaire in his seventies going up against another.
The prospect is not sitting well with Mr Bloomberg's five debate rivals, who are likely to hurl searing salvos at him when they take the stage in Las Vegas for a two-hour showdown starting 6pm local time.
"I got news for Mr Bloomberg," Senator Sanders, the left wing firebrand who won last week's New Hampshire primary, told voters in Nevada on Sunday. "The American people are sick and tired of billionaires buying elections."
Mr Bloomberg has essentially foregone the campaigning in the first four state-wide contests, including Nevada, which votes on Saturday.
Instead he is going all out on Super Tuesday, March 3, when 14 states including California and Texas vote on a Democratic nominee.
The ninth Democratic debate's other participants – former vice president Mr Biden, former South Bend, Indiana mayor Pete Buttigieg and Senator Amy Klobuchar, all moderates, and the more progressive senators Mr Sanders and Ms Warren – have criticised Mr Bloomberg for his campaign approach.
Ms Warren, whose campaign seems to be losing steam, has largely refrained from attacking other hopefuls on the debate stage. She now needs a boost, and badly.
On Tuesday she tweeted "it's a shame" that Mr Bloomberg can buy his way into the debate while she and others have spent so much time on the campaign trail.
Mr Bloomberg is said to be worth $55.5 billion (Dh203.8bn).
Mr Buttigieg also took aim at the billionaire on Tuesday when asked during a CNN town hall if he thought Mr Bloomberg was trying to buy his way in.
"What else do you call it when you dip into your endless reserves of millions and billions and don't go through the process of campaigning in states like Nevada, or Iowa, or New Hampshire?" he asked.
"Humbling yourself, going into the diners and the backyards, looking eye to eye to voters?"
On Tuesday, Mr Bloomberg’s spokesman Timothy O'Brien said the presidential hopeful would sell the company he founded and that made him a billionaire, Bloomberg, should his presidential campaign be successful.
Mr O'Brien accused the Sanders campaign of behaving in a "Trumpy" way by asserting, falsely, that Mr Bloomberg has had a heart attack.
He had stents installed in 2000 because of a coronary blockage. Mr Sanders did have a heart attack, in October.
"Those are the facts. It's a dangerous time when Sanders goes all in with Trumpism," Mr O'Brien tweeted.
Nationally Mr Bloomberg, 78, is surging. Two polls released on Tuesday show him leapfrogging rivals to claim second spot in the party nomination race behind Mr Sanders, with Mr Biden in third place.
Mr Buttigieg, who narrowly won Iowa and finished second in New Hampshire, will want to show viewers that his strong early performances were no fluke.
For Ms Warren, Ms Klobuchar and Mr Biden, who performed poorly in those two states, the Nevada debate is a critical opportunity for them to convince voters that they belong in the race.
For Mr Bloomberg, it could be his introduction to millions of voters unfamiliar with his 12-year stint as mayor of America's largest city.
Some of his policies as mayor are facing sharp criticism. Several rivals have highlighted the stop-and-frisk police operations during Mr Bloomberg's tenure, which disproportionately affected people of colour.
On Saturday, he acknowledged that he defended the policy "for too long" because he failed to understand the "unintended pain" it caused minority families.
"I should have acted sooner and faster to stop it," Mr Bloomberg said. "I didn't, and for that I apologised."
But that has not quelled the furore. "Sixty billion dollars can buy you a lot of advertising, but it can't erase your record," Mr Biden told broadcaster NBC on Sunday.