HOUSTON — Former Vice President Joseph R. Biden Jr. clung tightly to the legacy of the Obama administration in a Democratic primary debate on Thursday, asking voters to view him as a stand-in for the former president as an array of progressive challengers, led by Senators Elizabeth Warren and Bernie Sanders, brandished more daring policy promises and questioned Mr. Biden’s political strength.
Facing all of his closest competitors for the first time in a debate, Mr. Biden, the Democratic front-runner, repeatedly invoked President Barack Obama’s name and policy record as a shield against rivals who suggested his own record was flawed, or implied his agenda lacked ambition. On health care, immigration, foreign wars and more, Mr. Biden’s central theme was his tenure serving under Mr. Obama.
By constantly invoking Mr. Obama, a popular figure among Democrats, Mr. Biden sought to mute the ideological and generational divisions that have left him vulnerable in the primary race. To voters who might see him as a candidate of the past, Mr. Biden seemed to counter that the past was not so bad.
In an early exchange over health care, Mr. Biden referred to Ms. Warren’s support for Mr. Sanders’s “Medicare for all” plan. “The senator says she’s for Bernie,” Mr. Biden said. “Well, I’m for Barack — I think the Obamacare worked.”
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Explaining his preference for more incremental health care improvements, like the creation of an optional government-backed plan, Mr. Biden challenged Ms. Warren and Mr. Sanders to defend the cost of their plans, warning that they would require tax increases on middle-income Americans.
Mr. Biden was steadier in what was his third debate of the primary contest, rattling off statistics and parrying attacks with good cheer, though he still rambled at other moments. And despite their criticism, none of the nine other candidates onstage appeared to significantly damage his candidacy.
But Mr. Biden’s challengers were undeterred by his embrace of Mr. Obama, and the progressive candidates made clear the choice before primary voters.
Several of them argued — some subtly, some stridently — that the party needed to move well beyond the policies of the last Democratic president. And if Mr. Biden appealed to voters’ sense of nostalgia, his rivals pressed the case for broader change.
Ms. Warren, of Massachusetts, argued that her and Mr. Sanders’s approach would build upon Mr. Obama’s legacy rather than unraveling it. She credited Mr. Obama with having “fundamentally transformed health care,” but said the next president had to go further.
“The question is, how best can we improve on it?” Ms. Warren said, promising: “Costs are going to go up for giant corporations, but for hard-working families across this country, costs are going to go down.”
Mr. Sanders sidestepped the mention of Mr. Obama altogether, asserting to Mr. Biden that a single-payer system would save consumers money by breaking the influence of insurers seeking to “protect their profits.”
“Americans don’t want to pay twice as much as other countries,” Mr. Sanders, of Vermont, said.
Other challengers in the 10-candidate field were less diplomatic in demanding a break from the center-left policy framework that guided the Obama administration. Julián Castro, who served as Mr. Obama’s housing secretary, echoed the news anchor Jorge Ramos in questioning the Obama administration’s record of deporting millions of illegal immigrants. When Mr. Ramos pressed Mr. Biden to say whether he had made any mistakes on immigration as vice president, Mr. Biden pleaded personal loyalty.
“We didn’t lock people up in cages, we didn’t separate families,” Mr. Biden said, adding, “The president did the best thing that was able to be done.”
But pressed by Mr. Ramos about his own role, Mr. Biden deflected: “I’m the vice president of the United States,” he said. That prompted an impatient reaction from Mr. Castro, who repeated a criticism previously leveled at Mr. Biden by Senator Cory Booker of New Jersey.
“Every time something good about Barack Obama comes up, he says, ‘Oh, I was there, I was there, I was there, that’s me, too,’” said Mr. Castro, who was the most aggressive combatant against Mr. Biden on Thursday. “And then every time somebody questions part of the administration that we were both part of, he says, ‘Well, that was the president.’”
Mr. Biden, he said, “wants to take credit for Obama’s work, but not have to answer to any questions.”
Mr. Biden responded as if Mr. Castro was asserting that Mr. Biden was insufficiently defensive of the former president.
“I stand with Barack Obama all eight years,” Mr. Biden said. “Good, bad and indifferent.”
Mr. Biden’s fealty to Mr. Obama, throughout the debate, was consistent with his overall approach to the campaign. He has staked out a position, unapologetically, as a candidate of the Democratic center, building a sizable but far from dominant base of support, anchored in the admiration of moderates, older voters and African-Americans. But he has yet to expand his appeal beyond that base, which appears to make up between a quarter and a third of the Democratic electorate.
His resilience, though, has prompted some of his rivals to recalibrate their approach as the race enters the fall. After unleashing one of the contest’s toughest attacks against Mr. Biden in the first debate, Senator Kamala Harris of California steadfastly avoided critiquing the former vice president or any of her Democratic opponents.
Ms. Harris used her opening statement to speak directly to, and criticize, President Trump. During the health care contretemps she lamented that “not once have we talked about Donald Trump.” And when she made the case for using executive action to overcome legislative gridlock, she turned to Mr. Biden, let out a laugh and borrowed Mr. Obama’s signature line. “Hey, Joe, let’s say, ‘Yes we can,’” she said.
Ms. Harris’s attempt at a strategic makeover was hard to miss, but other candidates also tried to show voters a fuller version of themselves. Ms. Warren stayed true to her vision for sweeping policy proposals, but she also used the debate, held in the city where she went to college, to talk more about her personal story, which many voters are only dimly aware of. She recalled her Oklahoma youth, repeatedly cited her brothers’ military service and talked about being a public school teacher.
If Ms. Warren seemed determined to unfurl her biography, Mr. Sanders came prepared to take on Mr. Biden. The Vermont senator was assertive about drawing contrasts between his progressive credentials and Mr. Biden’s far more varied record. Where Mr. Sanders shied away from direct conflict when he shared a debate stage with Mr. Biden in June, this time he sought out areas of sharp disagreement, including over the NAFTA trade deal and the war in Iraq. Mr. Biden supported both; Mr. Sanders opposed them.
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The Democratic Party’s lively, sometimes heated internal disagreements were on vivid display throughout the night, on subjects as diverse as gun control, trade with China, the war in Afghanistan and the Senate filibuster. And if the clearest philosophical gulf separated Mr. Biden from the prominent populists who flanked him — Mr. Sanders and Ms. Warren — the debate made plain that the party was in the midst of a far more complex process of defining its identity.
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 few of the trailing contenders sharpened their attacks.
Senator Amy Klobuchar of Minnesota derided Mr. Sanders’s single-payer health care 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?”
A harshly contentious clash between Mr. Biden and Mr. Castro provided one of the most heated exchanges of the evening, at least in terms of political theatrics. Seizing on a moment in which Mr. Biden described his health care proposals imprecisely, Mr. Castro questioned Mr. Biden’s memory — a charged subject for the former vice president, who is 76.
“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.” (The former vice president did, however, show his fondness for a bygone era later in the forum when he dropped in a reference to record players.)
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.”
Mr. Booker, who lives in Newark, said the outrage over gun violence was long overdue. “I’m sorry that it had to take issues coming to my neighborhood or personally affecting Beto to suddenly make us demand change,” he said. “This is a crisis of empathy in our nation. We are never going to solve this crisis if we have to wait for it to personally affect us or our neighborhood or our community before we demand action.”
There were, though, plenty of lighter moments in Houston.
Mr. Booker, a vegan, assured viewers he would not push Americans away from eating meat.
“First of all I want to say: No,” he said, before adding with a touch of comedic timing, “I want to translate that into Spanish: No.”
Ms. Harris drew titters for saying Mr. Trump “reminds me of that guy in ‘The Wizard of Oz,’” adding that “when you pull back the curtain, it’s a really small dude.”
And Andrew Yang left many of his competitors onstage all but speechless when he used his opening statement to unveil a new gambit aimed at drawing attention to his proposal to offer Americans a universal basic income.
“My campaign will now give a freedom dividend of $1,000 a month for an entire year to 10 American families, someone watching this at home right now,” he said. “If you believe that you can solve your own problems better than any politician, go to yang2020.com and tell us how $1,000 a month will help you do just that.”