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The nationally televised debate was for many voters the first unscripted look at Bloomberg, a media mogul and former New York mayor whose campaign until now has been fuelled by hundreds of millions of dollars of self-funded television ads and carefully choreographed personal appearances.
Bernie Sanders, Elizabeth Warren, Amy Klobuchar, Joe Biden and Pete Buttigieg lined up to attack Bloomberg, accusing him of trying to buy his way into the White House and criticising his record on race and his history of sexist and misogynist comments.
"We’re running against a billionaire who calls women fat broads and horse-face lesbians," said Warren, a senator from Massachusetts. "And, no, I’m not talking about Donald Trump, I’m talking about Mayor Bloomberg."
"Democrats take a huge risk if we just substitute one arrogant billionaire for another," she added.
Bloomberg, who entered the race in November and is skipping the first four early voting states in February to focus on later nominating contests in March, said he did not inherit his money, but made it as a businessman.
"I’m spending that money to get rid of Donald Trump – the worst president we’ve ever had. And if I can get that done, it will be a great contribution to America and to my kids," he said.
Biden, the former vice president, said Bloomberg had not managed New York very well during his three terms as mayor and said stop and frisk had thrown "close to 5 million young black men up against the wall."
The debate comes at a pivotal time, three days before Nevada's presidential caucuses, the third contest in the state-by-state race to find a challenger to Trump in the Nov. 3 election.
Biden and Warren, in particular, face the do-or-die task of reigniting their campaigns after poor showings in Iowa and New Hampshire earlier this month.
Bloomberg, 78, has come under heavy criticism on the campaign trail recently as his poll numbers have surged and his entry into the race on March 3 - known as Super Tuesday, when 14 states vote - draws closer.
He has risen to No. 2 among Democrats behind Sanders, according to a Reuters/Ipsos national poll released on Tuesday.
Bloomberg, the billionaire former New York mayor making his first debate appearance in the race, faced criticism from all his rivals on the stage in Las Vegas:
The former South Bend, Indiana, mayor went after both Bloomberg and U.S. Senator Bernie Sanders.
"Most Americans don’t see where they fit if they have to choose between a socialist who thinks money is the root of all evil and a billionaire who thinks that money ought to be the root of all power. Let's put forward somebody who actually lives and works in the middle class neighbourhood in an industrial Midwestern city. Let's put forward somebody who's actually a Democrat."
"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. We can do better."
"The fact of the matter is has not managed his city very well when he was there. He didn't get a whole lot done. He had stop-and-frisk - throwing close to 5 million young black men up against the wall - and when we came along in our administration, President Obama and I said we're going to send a mediator to stop it."
"I have been told as a woman, as someone that maybe no one thought was still going to be standing up on this stage, but I am because of pure grit ... I've been told many times to wait my turn and to step aside, and I’m not going to do that now ... I think we need something different than Donald Trump. I think [you] don’t look at Donald Trump and say, 'We need someone richer in the White House'.”
"I’m a philanthropist who didn’t inherit his money, but made his money. I’m spending that money to get rid of Donald Trump – the worst president we’ve ever had. And if I can get that done, it will be a great contribution to America and to my kids.”