HOUSTON — The leading Democratic presidential candidates split sharply over the issue of health care in a debate on Thursday night, exposing the gulf between former Vice President Joseph R. Biden Jr.’s careful moderate politics and the transformational liberal ambitions of Senators Elizabeth Warren of Massachusetts and Bernie Sanders of Vermont.
Mr. Biden, the front-runner facing all of his closest competitors for the first time in a debate, quickly took the initiative to challenge Ms. Warren and Mr. Sanders for supporting a “Medicare for all”-style health care system, which would displace the existing forms of private insurance. Cloaking himself in the accomplishments of the Obama administration, Mr. Biden branded Ms. Warren as seeking to upend the progress of the Affordable Care Act.
“I know the senator says she’s with Bernie — well, I’m for Barack,” Mr. Biden jabbed, attacking the cost of a single-payer program: “How are we going to pay for it?”
Ms. Warren and Mr. Sanders, flanking Mr. Biden onstage, pushed back in tandem, dismissing Mr. Biden’s criticism and promising that a government-managed health insurance system would ultimately be less expensive for consumers than the private insurance they currently buy. Ms. Warren credited former President Barack Obama with having “fundamentally transformed health care in America” but said the country needed to go further.
“The richest individuals and the biggest corporations are going to pay more, and middle-class families are going to pay less,” she said.
But asked twice by a moderator whether she would acknowledge that the taxes of middle-income Americans would go up under her proposal, she declined to respond directly.
The back and forth between the three leading contenders over health care was a continuation of the discussion that dominated the first two debates and illustrated the fierce divisions in the party over what approach best honors Mr. Obama’s legacy — building upon the Affordable Care Act or replacing it entirely with a single-payer system.
In a tart exchange that channeled their profound philosophical differences, Mr. Sanders held up Canada as a country that provided universal coverage for a lower total cost, prompting Mr. Biden to jump in: “This is America.”
Mr. Sanders fired back: “Americans don’t want to pay twice as much as other countries.”
The remaining field of candidates arrayed themselves around the same philosophical dividing line, most of them aligning more closely with Mr. Biden. And for the first time in this primary race, a handful of the trailing contenders sharpened their attacks.
Senator Amy Klobuchar of Minnesota derided Mr. Sanders’s single-payer bill as a “bad idea,” while Mayor Pete Buttigieg of South Bend, Ind., accused Mr. Sanders and Ms. Warren of seeking to take away choice from consumers.
“I trust the American people to make the right choice for them,” Mr. Buttigieg said. “Why don’t you?”
But it was a harshly contentious clash between Mr. Biden and Julián Castro, the former federal housing secretary, that had the potential to stand out the most from the early exchanges of the evening, at least in terms of political theatrics. Seizing on a moment in which Mr. Biden appeared to reverse his own description of his health care proposals, Mr. Castro questioned Mr. Biden’s memory — a charged subject for the 76-year-old Democratic front-runner.
“Are you forgetting already what you said two minutes ago?” Mr. Castro said, prodding insistently before boasting, “I’m fulfilling the legacy of Barack Obama and you’re not.”
Mr. Biden shot back: “That would be a surprise to him.”
As the bickering grew more intense, Mr. Buttigieg interjected that voters would not like the attacks and attempts to “score points,” which he said prompted only more derision.
“That’s called a Democratic primary election,” Mr. Castro interjected. “This is what we’re here for.”
Trying to stay above the fray was the candidate who unleashed one of the race’s toughest attacks at the first debate, Senator Kamala Harris of California. Ms. Harris used her opening statement to speak directly to, and criticize, President Trump, and during the health care contretemps she lamented that “not once have we talked about Donald Trump.”
There was more consensus on the stage when it came to praising the leadership of former Representative Beto O’Rourke of Texas in the aftermath of the mass shooting last month in El Paso, his hometown. And Mr. O’Rourke won a booming ovation from the audience when he was asked whether he would try to confiscate some weapons.
“Hell, yes, we’re going to take your AR-15, your AK-47,” he said. “We’re not going to allow it to be used against fellow Americans anymore.”
Thursday’s debate, the first since July, came at a moment when a race that once seemed volatile had become remarkably stable. Over the summer the field divided into two distinct classes, with Mr. Biden, Mr. Sanders and Ms. Warren representing the top tier in both national and early-state polling.
While Mr. Biden remains the front-runner in the Democratic race, he has yet to produce the kind of commanding debate performance that might excite undecided Democrats or put to rest their unease about his readiness for a contest with Mr. Trump. Mr. Biden’s support has held steady for the past few months, hovering around 30 percent of primary voters, but there is no indication he has won over any skeptical voters since entering the race in April.
After a disastrous first debate in June, when Ms. Harris delivered a thumping denunciation of his record on race, Mr. Biden appeared somewhat more sure-footed during a July debate in Detroit.
The most significant change to the campaign over the course of the summer has been Ms. Warren’s steady rise — and the impact her surge has had on the trailing candidates. After months of training their fire at Mr. Biden, a few of the Democratic hopefuls have started to target the Massachusetts senator, specifically her array of ambitious policy proposals.
At the New Hampshire Democratic Convention last weekend, Mr. Booker and Mr. Buttigieg both invoked Ms. Warren’s much-discussed plans to make the case that ideas alone were insufficient to winning the presidency and enacting an agenda.
It was the first time either of the two candidates, who have needled Mr. Biden in ways that are both overt and subtle, had implicitly taken on Ms. Warren or reflected both her new status in the race and the urgency others are feeling to slow her rise.
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Though Ms. Warren has drawn the most attention as a potential antagonist for Mr. Biden, Mr. Sanders has shown a greater appetite than she has for direct conflict. On the campaign trail, he has periodically lashed Mr. Biden for having supported free-trade deals like NAFTA and the Trans-Pacific Partnership, and for opposing the creation of a single-payer health care system. Mr. Sanders is under political pressure, too: With Ms. Warren gaining strength and his own poll numbers stalled, he cannot afford to let her emerge as the leading champion of their party’s populist left. Should a Biden-Warren rivalry come to dominate the next phase of the race, Mr. Sanders could find himself frozen in third place.
But that is a better position than where most of the other contenders find themselves at the moment. And few were under as much pressure to show signs of progress as Mr. Booker and Mr. Buttigieg.
For Mr. Booker, demonstrating a sign of political life in Houston was a necessity, because it would be difficult for him to finance a full-fledged campaign into next year if he did not show growth in early-state polling. While he has several endorsements in both Iowa and New Hampshire, he has sought to straddle the mainstream and progressive wings of the party and has proved unable to break through the din of a crowded field.
The 38-year-old Mr. Buttigieg has similarly sought to avoid lining up with either ideological bloc and has not been able to sustain any real growth in the polls with his generational case. He is in no danger of being unable to meet a payroll, but he faces the specter of being branded as a mere donor candidate, beloved by the party’s wealthy financiers but with scant support from its rank-and-file.
For Ms. Harris, the Houston forum represented a chance to show Democrats the promise in her candidacy that was on display in the first debate, when she confronted Mr. Biden. But she has struggled in the months since to articulate a coherent case for her candidacy or even to settle on a single identifiable message. Like some of the contenders just on the other edge of the top tier, Ms. Harris has sought to avoid being seen as full-throated progressive or as a pragmatic moderate.
There is little doubt about Ms. Klobuchar’s positioning — she aligns with the party’s centrist wing. But she has proved unwilling to take on her fellow Democrats and has been stagnant in the polls, even in her neighboring state of Iowa where she has spent considerable time on the ground this year.
For Mr. Castro and Mr. O’Rourke, the debate offered at least a symbolic home-field advantage. The two Texans have held up their home state as something of a rationale for their campaigns: They have both presented themselves as experts on the culture of America’s border region and called for liberalizing the country’s immigration laws, with Mr. Castro positioned on the left flank of the field. And in the second debate in July, Mr. O’Rourke argued hopefully that if Democrats nominated him, they would have a good shot at carrying Texas and blowing a hole in the Republicans’ path to 270 Electoral College votes.
Mr. O’Rourke’s candidacy has changed drastically since then: After a white supremacist gunman carried out a massacre in El Paso last month, Mr. O’Rourke reoriented his campaign to focus on cracking down on gun violence and battling racism. He has pushed the Democratic debate leftward on the issue of gun control with his call to require the owners of assault-style weapons to sell them to the government.
The least predictable figure onstage was perhaps Andrew Yang, the former technology executive who has promised to establish a new government benefit to give every citizen $1,000 per month. Mr. Yang, who has never held public office, has built a small but sturdy constituency that has kept him in the debates longer than a throng of more conventionally qualified candidates, including sitting governors and senators. Whether his message of alarm about the automation of American jobs can resonate more widely remains to be seen.