Elizabeth Warren shamed Mike Bloomberg for allegedly calling women “fat broads.”

Amy Klobuchar asked Pete Buttigieg if he was calling her “dumb.”

Bloomberg shut down Bernie Sanders by quipping that "the best-known socialist in the country happens to be a millionaire with three houses."

The Democrats’ showdown Wednesday night in Las Vegas repeatedly put Bloomberg in his primary rivals’ crosshairs as the surging billionaire made his debate-stage debut. But the conflagration quickly expanded into an all-out melee and easily the most aggressive debate of the nomination season to date, as each of the candidates took sharp and often personal shots at one another.

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The fight started out white-hot when, in her first turn at the microphone, Warren asserted that she would support whoever wins the nomination, but warned that Democrats "take a huge risk if we just substitute one arrogant billionaire for another."

She said the field is running against somebody "who calls women fat broads and horse-faced lesbians -- and no, I’m not talking about Donald Trump, I’m talking about Mayor Bloomberg."

As some attendees cheered, she added, "We 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 like redlining and stop-and-frisk." Bloomberg strongly denied supporting redlining, or systematically denying services to certain neighborhoods.

Warren, after a disappointing fourth-place finish in New Hampshire, was particularly combative on stage as she tangled with multiple rivals in a bid to distinguish herself once more ahead of the Nevada caucuses this weekend.

In another explosive moment, attendees erupted in the hall when Warren called on Bloomberg to release women from nondisclosure agreements they have signed concerning their civil complaints that he harassed them in the workplace.

"Mr. Mayor, are you willing to release all of those women from those nondisclosure agreements, so we can hear their side of the story?" Warren asked, to thunderous applause.

"We have very few nondisclosure agreements," Bloomberg meekly responded. "None of them accused me of doing anything -- except, maybe they didn't like a joke I told." The audience then booed loudly.

"It was agreements between two parties that wanted to keep it quiet, and that's up to them. They signed those agreements, and we'll live with it," Bloomberg said.

The audience continued to boo Bloomberg as he repeatedly declined to waive the agreements on stage during the discussion, even after Biden yelled that it would be easy to do so. Bloomberg also did not respond to Warren's request that he specify exactly how many nondisclosure agreements he had arranged.

Amid calls to release his tax returns, Bloomberg drew more jeers by saying it "takes a long time" and that he "can't go to Turbo Tax," and that he would be releasing them "in a few weeks."

"We've heard that before," Amy Klobuchar mused, referring to the president. (Trump, after the debate ended, derided Bloomberg's performance as "perhaps the worst in the history of debates.")

Bizarrely, Klobuchar also added, "I have an idea of how we can stop sexism on the internet: We could nominate a woman for candidate for President of the United States."

Sanders, meanwhile, warned that Bloomberg couldn't be elected because he wouldn't be able to assemble a diverse coalition of voters.

He also called it "immoral" that Bloomberg had vast wealth while homelessness remained a problem. A debate moderator then asked a pointed question: "Mayor Bloomberg, should you exist?"

"Mr. Bloomberg had policies in New York City of stop-and-frisk that went after African American and Latino people in an outrageous way," Sanders said.

"I don't think there's any chance of the senator beating President Trump," Bloomberg responded flatly.

He would go on to slam communism, saying Sanders' apparent disdain for capitalism would lead to it and be catastrophic for Democrats in November. Sanders called the "communism" reference a "cheap shot," insisting that he is accurately described as a democratic socialist.

"What a wonderful country we have!" Bloomberg said. "The best-known socialist in the country happens to be a millionaire with three houses. What’d I miss here?”

“I work in Washington," Sanders began.

"That's the first problem," Bloomberg retorted.

Bloomberg also asserted that Americans simply don't want to lose their private health care plans under Sanders' "Medicare-for-all" initiative. That was a point that Pete Buttigieg argued later, as the debate quickly descended into a crossfire among the candidates: "We can actually deliver health care without taking it away from anyone," he said, during a discussion with Sanders.

Buttigieg added that Sanders' plans would cost "$50 trillion; he's only explained $25 trillion in revenue, which means the hole in there is bigger than the entire economy of the United States."

Sanders responded that Medicare-for-all would save $450 billion a year by reducing administrative costs -- a claim that numerous studies have disputed.

Buttigieg also reminded the audience that Bloomberg hasn't always been a Democrat, saying the party should nominate someone who at least "actually" belongs to the party. Bloomberg, for his part, repeatedly touted his experience as an executive and his ability to take on Trump, boasting: “I’m a New Yorker. I know how to take on an arrogant con man.”

But much of the debate was so centered on the Democratic primary candidates attacking each other that at one point Klobuchar pleaded, “We have not been talking enough about Donald Trump.”

The incumbent president merited passing scornful remarks from those on stage, but the attacks were largely directed inside the party tent.

Joe Biden later hammered Bloomberg on his stop-and-frisk policy, saying it "violated every right people have."

Warren picked up that point, accusing Bloomberg of "willful ignorance" and "shutting out" the sounds of protests on his own streets. "You need a different apology here, Mr. Mayor," Warren said, after Bloomberg said he regretted the implementation of stop-and-frisk.

Separately, Warren unexpectedly took aim at Klobuchar, who bested Warren in the New Hampshire primary. Her health care plan, Warren mocked, was "like a Post-It note: 'Insert plan here.'" The broadside prompted a stunned reaction in the debate hall.

"I must say I take personal offense, since Post-It notes were invented in my state," Klobuchar said in response, to laughter. She emphasized that her plan was to establish a public option. "You don't put your money on a number that's not even on the wheel," Klobuchar added, noting that "two-thirds of the Democratic senators are not on that bill," and many House members see the problems in "blowing up the Affordable Care Act."

"We need to improve the Affordable Care Act, not blow it up," Klobuchar said.

Biden interjected that he was the only person on the stage who had "gotten anything done" on health care, referring to the ACA.

Klobuchar also took heat for a recent interview, in which she was unable to name the president of Mexico. Buttigieg highlighting the vulnerability, noting that Klobuchar serves on several Senate committees that relate to Mexico.

"Are you trying to say I'm dumb?" Klobuchar responded, saying it was a temporary lapse in memory and nothing more. "I made an error. Sometimes people forget names."

Minutes later, Buttigieg attacked Klobuchar for voting to confirm Trump's nominee Kevin McAleenan to head Customs and Border Protection -- prompting Klobuchar to respond that McAleenan also had the support of many Democrats, including Obama administration officials.

"I wish everyone was as perfect as you, Pete," Klobuchar said, clearly frustrated. "You've memorized a bunch of talking points."

Sanders, in a calmer moment in the debate hosted by NBC and MSNBC, warned attendees that the world could soon be permanently damaged within seven years by climate change, repeating a concern that has been voiced for generations, often with similar deadlines from organizations like NASA and the United Nations that have come and gone. He pushed for a total fracking ban, a proposal that Republicans have said would only help Trump's chances to carry states like Pennsylvania.

Warren touted her concern for "environmental justice," saying her intersectional approach to climate change would also consider "communities of color."

The very end of the debate also provided a clear indication of how the bitter contest could be fought all the way through the convention. In a lightning round, only Sanders agreed that if no candidate receives a majority of delegates in the first round of balloting at the Democratic National Convention, then the candidate who receives a plurality should become the nominee.

Every other candidate suggested that the party's "process" should then take over, effectively signaling they would be open to so-called "superdelegates," or key party figures, potentially deciding the race in future rounds of balloting. In 2016, superdelegates played a role in the first round of balloting, but reforms to the rules championed by Sanders supporters will prevent that this time around.

Bloomberg was on the debate stage for the first time, after the Democratic National Committee (DNC) abruptly dropped the requirement that candidates meet an individual donor threshold to qualify. Bloomberg donated $300,000 to the DNC shortly before joining the presidential race last year.

“To now change the rules in the middle of the game to accommodate Mike Bloomberg, who is trying to buy his way into the Democratic nomination, is wrong,” Bernie Sanders adviser Jeff Weaver said after the debate rules were revealed. “That’s the definition of a rigged system.”

As Biden’s poll numbers have slid following his fourth- and fifth-place finishes in Iowa and New Hampshire, Bloomberg’s seen his poll numbers soar, thanks in part to roughly $400 million he’s spent to run ads across the country on TV, Facebook and Google.

While Bloomberg won't be on the ballot Saturday, he faced the expected intense scrutiny on national television for the first time, having faced relatively little in his surprisingly swift rise from nonpartisan megadonor to top-tier contender.

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On CNN early in the day, Sanders' national press secretary Briahna Joy Gray tried to rebut questions surrounding the Vermont senator’s health by pointing to Bloomberg, who she said had also “suffered heart attacks in the past.”

Sanders suffered a heart attack last fall and released letters from doctors attesting to his health. But Bloomberg has never suffered a heart attack; he released a doctors’ letter last year that said he did undergo coronary stent surgery in 2000.

Gray eventually walked back her statement, saying on Twitter that she “misspoke” about Bloomberg’s health.

Separately, the Biden campaign took on Bloomberg over ads that feature him working closely with former President Barack Obama. The Biden campaign posted a video on social media highlighting past comments Bloomberg made criticizing Obama on health care and climate change and accusing him of failing to address racism during his term.

The video also includes a clip of Bloomberg declaring “I’m a friend of Donald Trump’s, he’s a New York icon.”

Bloomberg is avoiding the earliest primary states, focusing instead on campaigning in the 14 states that vote in the March 3 Super Tuesday primaries. And his massive campaign — with over 2,000 staffers nationwide and over $400 million spent on ads already — has given him enough of a boost to win high-profile endorsements and double-digit support in the polls.

With much of the attention on Bloomberg, there is increasing fear from establishment-minded Democrats about Sanders' strength in the race. After he finished at or near the top in Iowa and New Hampshire, polls suggest the self-described democratic socialist is poised for another strong showing in Nevada's caucuses on Saturday.

The debate Wednesday did not make clear whether any in the rest of the field will emerge the clear alternative to Sanders come Super Tuesday.

As the showdown came to a close, Klobuchar remarked: "This has been quite a debate."

Fox News' Paul Steinhauser in Las Vegas and The Associated Press contributed to this report.