LAS VEGAS — The Democratic presidential candidates turned on one another in scorching and personal terms in a debate on Wednesday night, with two of the leading candidates, Senator Bernie Sanders and Michael R. Bloomberg, forced onto the defensive repeatedly throughout the evening.
In his first appearance in a presidential debate, Mr. Bloomberg, the former mayor of New York City, struggled from the start to address his past support for stop-and-frisk policing and the allegations he has faced over the years of crude and disrespectful behavior toward women. Time and again, Mr. Bloomberg had obvious difficulty countering criticism that could threaten him in a Democratic Party that counts women and African-Americans among its most important constituencies.
Two candidates who have shied away from direct conflict in past debates, Senator Elizabeth Warren of Massachusetts and former Vice President Joseph R. Biden Jr., mounted something of a tag-team onslaught against Mr. Bloomberg, several times leaving him visibly irked and straining to respond.
From the first seconds, when Mr. Sanders used the initial question to attack what he called Mr. Bloomberg’s “outrageous” policing record, it was clear that this debate would be far more heated than any of the previous forums. The unrelenting attacks reflected the urgency of the moment, as Mr. Sanders gains strength and those hoping to slow his candidacy are increasingly crowded out by Mr. Bloomberg and his unprecedented spending spree.
Ms. Warren landed the most stinging blows against Mr. Bloomberg throughout the debate, starting with an opening broadside that likened him to the figure most reviled among Democrats: President Trump.
“I’d like to talk about who we’re running against: a billionaire who calls women fat broads and horse-faced lesbians,” Ms. Warren said. “And no, I’m not talking about Donald Trump. I’m talking about Mayor Bloomberg.”
It was not only Mr. Sanders and Mr. Bloomberg who were subjected to withering criticism: Senator Amy Klobuchar of Minnesota and former Mayor Pete Buttigieg of South Bend, Ind., also engaged in a bitter and lengthy colloquy about foreign policy and their qualifications for the presidency, culminating in a sharp exchange in which Ms. Klobuchar asked Mr. Buttigieg if he was calling her “dumb.”
There was little in the debate to suggest that Mr. Sanders, the national front-runner and the favorite to win Nevada’s caucuses on Saturday, had been knocked off balance, and the pile-on against Mr. Bloomberg had the potential to work in Mr. Sanders’s favor by keeping the focus of hostilities elsewhere.
But Mr. Sanders, too, was pressed to address some of the persistent questions about his candidacy, including whether he would release a fuller version of his medical records and why his candidacy appears to inspire uniquely vitriolic behavior by some of his supporters on the internet. Mr. Sanders, Vermont’s junior senator, insisted that nearly all of his online fans were good and decent people, but said he would “disown those people” who behave in deplorable ways.
Nobody acted with more urgency than Ms. Warren, who finished a distant fourth in New Hampshire after doing little to stand out in the debate there. She repeatedly inserted herself into main currents of the conversation. The challenge for her, though, is that her newfound vigor came after tens of thousands of Nevadans had already cast their ballots in early voting.
It was Ms. Warren who initiated the exchange that may have damaged Mr. Bloomberg the most when she repeatedly demanded to know whether he would be willing to release some of the former female employees at his news media organization from the nondisclosure agreements they had signed. He declined to do so, calling the agreements “consensual,” and minimized the underlying complaints by suggesting that the women merely “didn’t like a joke I told.”
After pressing Mr. Bloomberg and leaving him flustered, but unable to coax him into releasing the women she said he had “muzzled,” Ms. Warren then broadened her attack.
“We are not going to beat Donald Trump with a man who has who-knows-how-many nondisclosure agreements and the drip, drip, drip of stories of women saying they have been harassed and discriminated against,” she said.
Before Mr. Bloomberg could even try to defend himself, Mr. Biden, who has seen the former New York mayor claim some of his support, gladly stepped in. “All the mayor has to say is, You are released from the N.D.A., period,” Mr. Biden said, his voice rising.
In what became, for both of them, their most energetic debate in months, Mr. Biden and Ms. Warren teamed up to confront Mr. Bloomberg about his record on policing, challenging his expressions of contrition about his years of strong support for invasive searches that disproportionately targeted black and Hispanic men. The unlikely duo wielded the same combination of indignation and inquisition that framed their argument about sexual harassment.
“It’s not whether he apologized or not, it’s the policy,” Mr. Biden said, accusing Mr. Bloomberg of discounting concerns raised by the Obama administration.
As in most of the tough exchanges of the night, Mr. Bloomberg defended himself only up to a point: He explained that he was focused on protecting New Yorkers’ “right to live,” and in the process embraced a policing strategy he later came to regret. Looking back on his time as mayor, Mr. Bloomberg said, “the one thing I’m really worried about, embarrassed about, was how it turned out with stop and frisk.”
Ms. Warren jumped in to dissect that answer. “This isn’t about how it turned out,” she said. “This is about what it was designed to do, to begin with. It targeted communities of color.”
Mr. Bloomberg has risen in the polls thanks to spending more than $400 million in advertising, nearly all of it on next month’s Super Tuesday contests, but he had largely avoided engaging with voters, let alone his Democratic rivals. His decision to participate in the debate before those March 3 states was a gamble — and one his own campaign all but conceded they didn’t win.
They acknowledged his lackluster performance, at least in the first hour of the two-hour forum, even as they boasted about how all of the attacks on him demonstrated the threat he poses. “It took him just 45 minutes in his first debate in 10 years to get his legs on the stage,” Kevin Sheekey, Mr. Bloomberg’s campaign manager, said in a statement after the debate.
If Mr. Sanders appeared relatively unbruised Wednesday night, it was not clear that he did anything to put to rest the persistent reservations in the party about his prospects in a contest against Mr. Trump. He dismissed a moderator’s question about polling that found Americans deeply wary of socialism as a political label, noting that the same polling found him leading Mr. Trump in a general election.
Mr. Buttigieg and Mr. Bloomberg picked away at Mr. Sanders, with Mr. Bloomberg declaring that he would be a surefire loser in the general election and Mr. Buttigieg warning that it would be dangerous to nominate someone who “wants to burn the house down.”
After intraparty politesse prevailed in the first eight debates, when the harshest remarks onstage were usually reserved for Mr. Trump, the evident contempt some of these six candidates have for one another rang out like a jackpot in a slot machine on Wednesday. At no time was that more clear than when Ms. Klobuchar, who split the support of many moderate voters in New Hampshire with Mr. Buttigieg, was reminded by the former South Bend mayor that in a recent interview she had been unable to name the president of Mexico.
“You’re staking your candidacy on Washington experience,” Mr. Buttigieg said, pointing out that all of the committees she serves on involve Mexico.
“Are you trying to say I’m dumb — are you mocking me, Pete?” Ms. Klobuchar shot back, clearly stung. She then noted she had won all of her campaigns, while he had lost his sole statewide bid “by over 20 points.”
Later, Ms. Klobuchar said Mr. Buttigieg had simply “memorized a bunch of talking points” and had never been “in the arena.”
Both flashed sharp irritation, as Mr. Buttigieg criticized Ms. Klobuchar’s support for certain Trump appointees and Ms. Klobuchar described his political accomplishments as minimal.
“You don’t have to be in Washington to matter,” Mr. Buttigieg said, pointing to his tenure in the struggling city he sought to revive.
At one point, Ms. Klobuchar answered Mr. Buttigieg with blunt sarcasm: “I wish everyone was as perfect as you, Pete,” she said.
Yet the debate underscored the challenge facing candidates like Mr. Buttigieg and Ms. Klobuchar, who are hoping to ride a sense of momentum from Iowa and New Hampshire into stronger finishes elsewhere. But the two rivals spent much of the evening arguing with each other, and both were repeatedly upstaged by clashes involving the candidates better known at the national level.
Both were at risk of emerging from the debate weaker than they entered it, and at the end of the evening they had not clearly strengthened their claims to leadership of moderate forces in the Democratic coalition.
Just as contentious were the exchanges between Mr. Bloomberg, the proud billionaire, and Mr. Sanders, the democratic socialist who has said billionaires should not exist. Mr. Sanders, who had a heart attack in the fall, answered a question about his personal health records by noting that Mr. Bloomberg has “two stents as well,” prompting Mr. Bloomberg to say that his had been inserted two decades ago.
And after Mr. Bloomberg said he made no apologies for his wealth because he had worked hard for his money, Mr. Sanders interjected: “Maybe your workers played some role in that as well.”
Mr. Bloomberg eventually confronted Mr. Sanders, saying it was “ridiculous” to suggest the country would “throw out capitalism.”
Turning even more personal, Mr. Bloomberg said, “What a wonderful country we have — the best known socialist in the country happens to be a millionaire with three houses.”
After listing his residences, Mr. Sanders turned back to Mr. Bloomberg and asked: “Which tax haven do you call home?”
The sparring was hardly unanticipated. The rivalry between Mr. Sanders and Mr. Bloomberg has turned harshly personal this week, as their campaigns escalated a feud that both see as serving their political interests. On the morning of the debate, aides to Mr. Bloomberg and Mr. Sanders were trading slashing criticism about the health of the two men, who are both 78 years old, and transparency about their medical histories.
And in the days leading up to the debate, a number of the candidates denounced the personal attacks that Sanders supporters aimed at the female leaders of the influential union of Las Vegas’s casino employees, the culinary workers’ union, criticism that arose again at Wednesday’s debate.
If the other contenders are not able to slow Mr. Sanders in Nevada, he may gain enough momentum going into the Super Tuesday contests on March 3, when 15 states and territories will vote, to eventually claim the nomination. But if he falters here, it could throw the race open and create an opportunity for one or more of his rivals to assert themselves.
Mr. Sanders’s prominence in the race even drew a broadside from Ms. Warren, who has largely avoided tangling with him since entering the race.
“His campaign relentlessly attacks everyone,” she said, alluding to his supporters’ scorn for those who do not support “Medicare for all.” And then Ms. Warren made a reference to recent comments by Representative Alexandria Ocasio-Cortez of New York, who said that Mr. Sanders might not be able to initially pass single-payer insurance. “His own advisers say, ‘Eh, probably won’t happen, anyways,’” she said.
Mr. Sanders, though, did not return fire. And at the end of the evening, it was clear he had a larger goal in mind: claiming the Democratic nomination, even if he has accrued a plurality, but not a majority of delegates, by the end of the primary season.
“I think that the will of the people should prevail, yes,” he said. “The person who has the most votes should become the nominee.”