HOUSTON — The leading Democratic presidential candidates split sharply over the issue of health care in a debate Thursday night, exposing the gulf between former vice president Joe Biden’s careful moderate politics and the transformational liberal ambitions of Senators Elizabeth Warren of Massachusetts and Bernie Sanders of Vermont.
Biden, facing all of his closest competitors for the first time in a debate, quickly took the initiative to challenge Warren and Sanders for supporting a “Medicare for All”-style health care system that would displace the existing forms of private insurance. Cloaking himself in the accomplishments of the Obama administration, Biden branded 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,” Biden jabbed, attacking the cost of a single-payer program: “How are we going to pay for it?”
Warren and Sanders, flanking Biden onstage, pushed back in tandem, dismissing 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. 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.
In a tart exchange that channeled their profound philosophical differences, Sanders held up Canada as a country that provided universal coverage for a lower total cost, prompting Biden to jump in: “This is America.”
Sanders fired back: “Americans don’t want to pay twice as much as other countries.”
At one point, Biden called Sanders a ‘‘socialist,’’ and questioned the amount of time it would take to pass his more far-reaching plan. ‘‘Do something now!’’ Biden shouted.
The debate quickly developed as a virtual battle over the soul of the Democratic Party, including key questions of whether the party should pursue policies of sweeping change or a more incremental return to normalcy in the wake of Trump.
Sanders and Warren largely joined forces to spar with Biden early in the race, casting their plans as more properly suited to the major problems of the day.
The remaining field of candidates arrayed themselves around the same philosophical dividing line, most of them aligning more closely with 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 Sanders’ single-payer bill as a “bad idea,” while Mayor Pete Buttigieg of South Bend, Ind., accused Sanders and Warren of seeking to take away choice from consumers.
“I trust the American people to make the right choice for them,” Buttigieg said. “Why don’t you?”
But it was a harshly contentious clash between 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 Biden appeared to reverse his own description of his health care proposals, Castro questioned Biden’s memory — a charged subject for the 76-year-old Democratic front-runner.
“Are you forgetting already what you said two minutes ago?” Castro said, prodding insistently before boasting, “I’m fulfilling the legacy of Barack Obama and you’re not.”
Biden shot back: “That would be a surprise to him.”
As the bickering grew more intense, 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,” 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. Harris used her opening statement to speak directly to, and criticize, Trump and during the health care contretemps lamented that “not once have we talked about Donald Trump.”
The debate was an opportunity for Buttigieg to bring up his proposal to seek an authorization for the use of military force with a built-in three-year sunset that Congress would be required to renew.
‘‘We have got to put an end to endless war,’’ Buttigieg said.
Buttigieg also said that Trump treats ‘‘troops as props, or worse, tools for his own enrichment.’’ That final dig was an allusion to the Trump administration’s rerouting of US military personnel to overnight stays at his Trump Turnberry golf resort in Scotland.
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 Biden, Sanders and Warren representing the top tier both in national and early-state polling.
While 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 Trump. 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 Harris delivered a thumping denunciation of his record on race, Biden appeared somewhat more sure-footed during a July debate in Detroit. But he still struggled to turn back sharp critiques of his record from Harris and Senator Cory Booker of New Jersey, or to articulate a clear message for his own candidacy besides resisting the large-scale promises of the left.
The most significant change to the campaign over the course of the summer has been Warren’s steady rise — and the impact her surge has had on the trailing candidates. After months of training their fire at Biden, a few of the Democratic hopefuls have started to target the Massachusetts senator, specifically her array of ambitious policy proposals.
Sanders is under political pressure, too: With 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, Sanders could find himself frozen in third place.
For 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.
Buttigieg, 38, 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 Castro and former representative Beto O’Rourke of Texas, 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 Castro positioned on the left flank of the field.
After a white supremacist gunman carried out a massacre in his hometown, El Paso, last month, 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.
In the early going of the debate, O'Rourke claimed the perpetrator of last month’s mass shooting in El Paso was ‘‘inspired to kill by our president.’’
Trump campaign communications director Tim Murtaugh replied on Twitter that O'Rourke was ‘‘as desperate as he can be.’’
O’Rourke won a booming ovation from the Democratic 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.”
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.
Whether his message of alarm about the automation of American jobs can resonate more widely remains to be seen.
Material from the Washington Post and the Associated Press was used in this report.