DETROIT — Former Vice President Joseph R. Biden Jr. delivered a steadfast defense of his moderate policies in the Democratic primary debate on Wednesday, striking back at a familiar adversary, Senator Kamala Harris, but facing intensifying attacks on his record from liberal rivals including Senator Cory Booker and Julián Castro, the former housing secretary.
Mr. Biden, the leading candidate in the Democratic presidential race, entered the debate under pressure to articulate a more forceful rationale for his campaign and turn back attacks from his fellow Democrats, after failing to do so in his clash with Ms. Harris in the first debate in June.
In a handful of moments, Mr. Biden did just that, delivering pointed critiques of Ms. Harris and other challengers. But it was unclear by the end of the forum whether he was any closer to allaying liberals’ reservations about his candidacy, or inspiring a Democratic Party that is eager to defeat President Trump but has shifted to the left in the years since Mr. Biden served as vice president under Barack Obama. Though he may have won sympathy from Democratic voters for absorbing so many blows, he did not deliver a commanding performance to reclaim firm control of the race.
And in a sign of the party’s drift, Mr. Biden was repeatedly forced to defend not only his own record but also was questioned sharply about policies of Mr. Obama on issues such as immigration and trade.
Where ideology framed the conversation, the divisions resembled a mirror image of the dynamics that governed the first Democratic debate this week. On Tuesday, Senators Elizabeth Warren and Bernie Sanders, the populist liberals who have largely defined their party’s issue agenda, locked arms to deflect attacks from a gang of moderate underdogs, including Gov. Steve Bullock of Montana and former Representative John Delaney of Maryland. In the second debate, it was Mr. Biden and his relatively centrist values under collective assault.
Taken together, though, the back-to-back debates only threw the party’s factions into stark relief while delivering little clarity about the direction of a race that features well-funded candidates from its populist and moderate wings as well as a handful of contenders trying to straddle the divide.
In the opening moments of the debate, Mr. Biden took particular aim at Ms. Harris, accusing her of peddling “double talk” on health care and insisting that a range of liberal plans to displace the private health insurance system were too disruptive and too costly. He chided Ms. Harris for her proposal of a decade-long transition to a version of single-payer health care, urging voters to be skeptical “anytime somebody tells you you’re going to get something good in 10 years.”
“My response is: Obamacare is working,” said Mr. Biden, who has proposed the creation of an optional, government-backed health insurance plan.
Ms. Harris, on defense for the first time against Mr. Biden, insisted that her plan would do far more than his to ensure universal coverage: “Your plan, by contrast,” she retorted, “leaves out almost 10 million Americans.”
Yet by the end of the debate, Mr. Biden was besieged, attacked from all sides on a plethora of subjects including health care, immigration, trade, criminal justice, climate change, women’s rights and the war in Iraq. As he did at times in the first debate, he cut some of his answers short and stumbled over lines. And he flashed his impatience with rivals, like Mr. Booker and Ms. Harris, who he said were harrying him over events that occurred “a long, long time ago.”
Mr. Booker, among others, insisted that Mr. Biden’s half-century record was entirely fair game. Pointing to Mr. Biden’s history of shepherding harsh criminal-justice bills into law, Mr. Booker questioned whether the former vice president could lead the country forward on that and other contentious issues.
“This is one of those instances where the house was set on fire and you claimed responsibility for those laws,” Mr. Booker said. “And you can’t just now come out with a plan to put out that fire.”
While Mr. Biden found himself facing the most insistent attacks, Ms. Harris also came under fire and did not appear as steady as she did in the first debate, which breathed new life into her campaign. On Wednesday, it was Mr. Booker who often appeared to be the more formidable alternative to the front-runner.
If Tuesday’s debate defined the philosophical gulf within the Democratic Party, the Wednesday debate played out as a more complicated and personality-driven affair, featuring layers of political feuds and interlocking arguments over policy and electoral strategy.
At several early moments in the debate, some candidates onstage exhorted Democrats to keep their attention on President Trump and the Republican Party, and especially on their hard-line immigration policies and efforts to dismantle the Affordable Care Act. As Mr. Biden and Ms. Harris battled over the idea of “Medicare for all,” Senator Kirsten Gillibrand trained her fire on Republicans whose “whole goal is to take away your health care.”
In the midst of another Biden-Harris duel, midway through the debate, Senator Michael Bennet of Colorado erupted in impatience with the two of them for once again “debating what people did 50 years ago with busing” — the subject of Ms. Harris’s searing confrontation with the former vice president in June.
“Our schools are as segregated today as they were 50 years ago,” Mr. Bennet said. “We need a conversation about what’s happening now.”
And in an extended, contentious discussion of immigration, several Democrats tried to shift attention away from their own disagreements and toward the policies of the Trump administration.
“We can no longer allow a white nationalist to be in the White House,” said Gov. Jay Inslee of Washington, adding, “We have to make America what it’s always been: a place of refuge.”
But Mr. Inslee, too, took aim at Mr. Biden, describing himself as the only person onstage to have voted against the Iraq war — Mr. Biden took the opposite position — and branding Mr. Biden’s climate platform as an insufficient answer to a planetary crisis.
The most protracted clashes of the evening concerned criminal justice and immigration, and put several candidates besides Mr. Biden on the defensive. Attempting to pre-empt liberal attacks on his immigration record, Mr. Biden went on offense against Mr. Castro — the most vocal advocate for liberal immigration policy in the Democratic field — noting that he could not recall the former San Antonio mayor criticizing the Obama administration’s border policies when he was serving in the cabinet.
“If you cross the border illegally, you should be able to be sent back; it’s a crime,” said Mr. Biden, rejecting Mr. Castro’s plan to decriminalize illegal immigration.
Mr. Castro shot back that “it looks like one of us has learned the lessons of the past and one of us hasn’t,” and added that the only element missing in border policy is “politicians who have some guts.”
“I have guts enough to say his plan doesn’t make sense,” Mr. Biden retorted.
Still, the former vice president found himself fending off multiple attacks on the aggressive deportation policies of Mr. Obama, forcing him to choose between whether to defend a former president beloved by Democrats or align himself with the more liberal party of 2019. Pressed by Mayor Bill de Blasio of New York City to say whether he had tried to halt large-scale deportations when he was vice president, Mr. Biden evaded the question, saying he would not reveal his private conversations with Mr. Obama.
“You need to be able to answer the tough questions,” said Mr. de Blasio, in a jab that became something of a regular refrain throughout the evening.
But Mr. Biden’s position grew even more uncomfortable when Mr. Booker, standing just to his right, said: “You invoke President Obama more than anybody in this campaign. You can’t do it when it’s convenient and then dodge it when it’s not.”
Mr. Biden did come prepared for a clash over criminal justice and took aim at Mr. Booker’s tenure as mayor of Newark. “In 2007 you became mayor and you had a police department that was — you went out and hired Rudy Giuliani’s guy and engaged in stop and frisk,” he said.
Mr. Booker, however, had a counterattack planned. “You’re dipping into the Kool-Aid and you don’t even know the flavor,” he said, drawing laughter and applause.
Repeatedly highlighting the hard-line crime bill Mr. Biden wrote in the 1990s, Mr. Booker said: “There are people right now in prison for life for drug offenses because you stood up and used that tough on crime phony rhetoric that got a lot of people elected but destroyed communities like mine.”
Perhaps the only attack Mr. Biden deflected more or less easily was also among the most personal charges of the night, when Ms. Gillibrand cited an opinion article he wrote in 1981 criticizing a bill offering high income families tax credits for day care. Ms. Gillibrand invoked a line from the piece to claim that he had argued that mothers working outside the house could lead to “deterioration of the family.”
But Ms. Gillibrand had signaled days earlier that she would raise the topic and Mr. Biden denied that was his view, and quickly shifted the discussion to his own experience raising his two sons as “a single father” following his first wife’s death in a car accident. When Ms. Gillibrand continued to demand an answer about his position, Mr. Biden smiled, recalling the praise she had previously lavished on him.
“I don’t know what happened except you’re now running for president,” he said, to laughter from the crowd.
Mr. Biden also got a respite when Representative Tulsi Gabbard took aim at Ms. Harris’s record as a prosecutor in California.
“She put over 1,500 people in jail for marijuana violations and then laughed about it when she was asked if she ever smoked marijuana,” Ms. Gabbard said of Ms. Harris. “She blocked evidence that would have freed an innocent man from death row until the courts forced her to do so.”
Ms. Harris defended her record as state attorney general, arguing that her criminal justice overhaul became “a national model for the work that needs to be done.”
The friction over criminal justice, like other issues, seemed to make some candidates onstage uneasy. Andrew Yang, a former technology executive who has urged Democrats to treat the automation of working-class jobs as an urgent crisis, repeatedly nudged his fellow candidates away from their messiest confrontations.
“I speak for just about everyone watching when I say I would trust anyone on this stage much more than I would trust our current president on matters of criminal justice,” Mr. Yang said.
Far from resolving anything, the debate seemed to showcase just how messy and protracted the Democratic race could be. If Mr. Biden’s first debate held out the possibility of a rocket-like ascent by Ms. Harris, this one may have presaged a different trajectory for the race — one that has the former vice president persistently unable to quell resistance on the left, but with no singular rival emerging anytime soon as a focal point for that resistance.
Alexander Burns reported from Detroit, and Jonathan Martin from New York. Matt Stevens contributed reporting from New York.