Rising White House hopeful Michael Bloomberg has been taking heavy fire from fellow Democratic rivals hours ahead of a live TV debate.
The forum in Nevada is the first major test for the former New York City mayor after a late entry to the race.
The tycoon is under fierce attack over everything from his wealth and health to past remarks on race and gender.
The eventual Democratic nominee will face President Donald Trump, a Republican, in November's election.
Also on stage in Wednesday night's debate in Las Vegas will be Vermont Senator Bernie Sanders, former US Vice-President Joe Biden, Minnesota Senator Amy Klobuchar, Massachusetts Senator Elizabeth Warren and former South Bend, Indiana, mayor Pete Buttigieg.
A new Washington Post-ABC poll on Wednesday showed Mr Sanders at 32% with Mr Biden falling to 16%. Mr Bloomberg was third on 14% and Ms Warren fourth on 12%.
The attacks from Mr Bloomberg's Democratic rivals suggest it will be a tough night for the world's ninth richest man.
Sanders v Bloomberg
Mr Sanders took a direct swipe at the billionaire on Tuesday night, railing against Mr Bloomberg and "a system that allows billionaires to buy elections".
"My friends, that is called oligarchy, not democracy," Mr Sanders said.
Mr Bloomberg hit back at the self-described democratic socialist, comparing Mr Sanders' tactics to President Trump's.
His campaign has also begun to raise alarm that Mr Sanders will soon accumulate an unbeatable lead in delegates - required to win the Democratic nomination - if the field remains split by Mr Bloomberg's fellow moderates, Mr Biden, Mr Buttigieg and Ms Klobuchar.
The two campaigns have also sparred over their respective candidate's health records.
Defending Mr Sanders' decision not to release additional medical records after his heart attack in October, Sanders press secretary Briahna Joy Gray claimed that Mr Bloomberg had suffered heart attacks.
The businessman's campaign manager called the statement from Mr Sanders' team an "absolute lie". Ms Gray later said on Twitter that she "misspoke".
If either Mr Bloomberg or Mr Sanders - both 78 - were to win the presidency, he would become the oldest ever occupant of the White House.
Biden v Bloomberg
On Wednesday, Mr Biden attacked Mr Bloomberg's campaign ads touting his relationship with former President Barack Obama and his past party affiliation.
"The truth is he's basically been a Republican his whole life," said Mr Biden, who was Mr Obama's vice-president. "The fact of the matter is he didn't endorse Barack or me when we ran. This is a guy... using Barack's pictures like, you know, they're good buddies."
Mr Biden's suggestion that Mr Bloomberg's relationship with Mr Obama was more tenuous than the ads suggest has been echoed by others in the Obama administration.
"It's jarring to see all these Bloomberg ads that suggest Obama has endorsed him, especially considering how... perfunctory his endorsement of Obama was back in 2012," said former Obama spokesman Tommy Vietor on Twitter.
President Barack Obama is someone I was proud to call Commander-in-Chief, and someone I was honored to work with to get big things done.— Mike Bloomberg (@MikeBloomberg) February 5, 2020
I miss that feeling. pic.twitter.com/CNUWTzXtcU
Trump v Bloomberg
On Wednesday, President Trump renewed his attacks on his fellow septuagenarian New York billionaire.
"Is corrupt Bloomberg News going to say what a pathetic debater Mini Mike is," Mr Trump tweeted.
Mr Bloomberg tweeted back: "Impeached president says what?"
After Mr Bloomberg launched his run for president last November, the news service that bears his name said it would not investigate the billionaire or any of his Democratic rivals.
Mr Bloomberg is not competing in the first four Democratic leadership ballots this year, instead banking on a strong showing in the so-called Super Tuesday contest on 3 March, when 14 states will vote.
He has so far made up for his belated entry with a spending blitz, pouring more than $400m (£310m) already on national ads.