The other five candidates on stage in Las Vegas ripped into the former New York City mayor over his wealth, his record, his treatment of women and decades' worth of comments that don't quite align with where the Democratic Party seems to be at the moment.
"Democrats take a huge risk if we just substitute one arrogant billionaire for another," said Sen. Elizabeth Warren, who was coming off of two disappointing finishes in the first states voting.
Sen. Bernie Sanders, who got the most votes in both Iowa and New Hampshire, said Bloomberg brought policies to New York City that "went after African American and Latino people in an outrageous way."
Sen. Amy Klobuchar bristled at the Bloomberg campaign's suggestion that other moderates exit the race to make room for him.
"I don't think you look at Donald Trump and say, we need someone richer in the White House," she said.
Bloomberg sought to bring the fight to Sanders, echoing concerns raised by others in the field about what it would mean to have a self-described "Democratic socialist" at the top of the ticket this fall.
"I don't think there's any chance of the senator beating President Trump," Bloomberg said. "And if he goes and is the candidate, we will have Donald Trump for another four years, and we can't stand that."
None of the Democrats would quarrel with that last sentiment. The debate reflected anxieties and complicated calculations about a voting season that just started two weeks ago, but that could become essentially a two-person race as soon as early March.
Former Mayor Pete Buttigieg warned that, after Super Tuesday, only Sanders and Bloomberg -- "the two most polarizing figures on this stage" -- could be standing as viable candidates.
"We shouldn't have to choose between one candidate who wants to burn this party down and another who wants to buy this party out," Buttigieg said.
Bloomberg has been able to buy enough ads to make his arguments without significant refutation. That changed Wednesday night, and if he is pitching himself around electability, none of his rivals are buying it.
He seemed flustered by a series of attacks by Warren, who pointed out that his company signed non-disclosure agreements after negotiating settlements with women. Bloomberg wouldn't say how many women are covered under such agreements, and repeatedly declined to say he would release them to allow them to tell their stories.
"I hope you heard what his defense was: 'I've been nice to some women.' That just doesn't cut it," Warren said. "We need to know exactly what's lurking out there."
Former Vice President Joe Biden mocked the notion that the women in question want to remain silent.
"You think the women in fact were ready to say, 'I don't want anybody to know about what you did to me?' That's not how it works," he said.
Bloomberg insisted that he had no reason to change the terms of any agreements.
"They were made consensually and they have every right to expect that they will stay private," he said to scattered boos in the debate hall.
Bloomberg is trying to square himself up against Sanders. And Sanders -- now the national polling front-runner, and the only candidate other than Bloomberg with the resources to compete nationwide now -- also took significant incoming fire.
Buttigieg raised questions about the 78-year-old's age, as well as the costs of his plans.
"I'm actually less concerned about the lack of transparency on Sanders' personal health than I am about the lack of transparency on how to pay for his health care plan," he said.
Warren, whose policies most closely mirror Sanders', attacked virtually all of her rivals on policy grounds.
Of Buttigieg's health care proposal, she said, "It's not a plan, it's a PowerPoint. And Amy (Klobuchar)'s plan is even less. It's like a Post-It Note, insert plan here."
The scattered nature of the attacks may seem off for the moment, particularly given the stakes Democrats feel in seeking to defeat Trump.
But the candidates' urgency is driven by a brutal calendar and stubborn party rules that make it hard to catch up once a candidate falls behind in the delegate count. The next 10 days bring yet another debate as well as voting in the final two early contests -- Nevada and South Carolina -- after which the campaign goes national in a flash.
At the moment, Sanders and Bloomberg -- two vastly different white men of the same age -- appear to be the candidates who seem best positioned to stay in the race for as long as delegates are awarded. As they were quick to remind viewers, they see themselves and the country differently.
"Mike Bloomberg owns more wealth than the bottom 125 million Americans," Sanders said. "That's wrong. That's immoral. That should not be the case when we have half a million people sleeping out on the streets."
Bloomberg countered by attacking Sanders' plan for a wealth tax.
"I can't think of a way to make it easier for Donald Trump to get reelected than listening to this conversation," he said. "It's ridiculous. We're not going to throw out capitalism."
Toward the end of the debate, the candidates showed some cards that could prove vital -- and might speak to their confidence at the moment. Asked whether the leader in delegates should get the nomination -- even if he or she doesn't have an outright majority -- five of the six candidates said not necessarily, raising the possibility of power brokers swinging a convention to someone else.
Sanders was the outlier.
"I think the will of the people should prevail," he said.
This report was featured in the Thursday, Feb. 20, 2020, episode of “Start Here,” ABC News’ daily news podcast.
"Start Here" offers a straightforward look at the day's top stories in 20 minutes. Listen for free every weekday on Apple Podcasts, Google Podcasts, Spotify, the ABC News app or wherever you get your podcasts.