Senator Bernie Sanders jabbed his finger in the air. He left Joseph R. Biden Jr. resorting to misstatements to defend his record on bankruptcy legislation. He instructed viewers to “go to the YouTube” to check out the former vice president’s record themselves.
It was not the kind of debate performance suggesting a candidate ready for reconciliation.
After delivering a speech last week that appeared aimed at de-escalating the primary battle against Mr. Biden, Mr. Sanders laced into his opponent on Sunday night with a vigor that signaled that the fight — if not for delegates, then for the soul of the Democratic Party — was not over.
Under intense pressure to win nominating contests and already grappling with the effects of the coronavirus on the election, Mr. Sanders and his aides are now struggling to reconcile a sense of resignation and a desire to press on.
Yet even as his path to the nomination looks increasingly bleak, with Mr. Biden favored in three more primaries on Tuesday, some allies are publicly urging Mr. Sanders to remain in the race, eager for him to continue to pitch voters on a liberal agenda that he has promoted for decades.
“I want the senator to stay in,” Nina Turner, one of the campaign’s national co-chairs, said in an interview on Monday. “I think other voters have a right to have a choice. This is not a coronation. We know what happened last time in 2016 — it gave us Donald J. Trump.”
This campaign, she added, was “the culmination of this man’s life’s work.”
The decision on whether to drop out or continue the campaign is one that ultimately rests with Mr. Sanders and his wife, Jane Sanders, allies and friends say. His inner circle has only grown smaller in recent days, and hardly anyone on his campaign is willing to speculate where he may land.
Some Sanders aides and supporters privately acknowledge that his chances are daunting if not impossible given the broad coalition of Democrats that Mr. Biden appears to have assembled, and Mr. Sanders’s inability to bring new voters under his tent. But they describe Mr. Sanders as a fighter, and a headstrong one, whose instinct will be to keep battling. They also see an advantage in continuing to accrue delegates to use as leverage in negotiations with Mr. Biden over potential policy concessions.
To several Sanders aides and allies, the aggressiveness their candidate showed on the debate stage was a way of holding Mr. Biden accountable on the issues Mr. Sanders cares deeply about, perhaps fueled by the faint hope that he can persuade voters that he remains the best choice to consolidate the party and defeat President Trump.
“Even though Joe Biden is ahead in delegates at the moment, he is far from unifying the party,” Jeff Weaver, a longtime Sanders adviser, said after the debate. “And he hasn’t made the case that he can provide the kind of inspired leadership necessary to beat Donald Trump.”
Still, the campaign has not made a major television ad buy since last Wednesday, when it added about $2 million worth, though it did add another $250,000 worth on Thursday, spread across three states originally scheduled to vote on Tuesday: Florida, Illinois and Ohio (on Monday Ohio moved to postpone its primary). And Mr. Sanders risks looking out of step with the times if he persists in extending the primary against long odds, while the nation is in the middle of a public health crisis.
Even some of his biggest supporters suggested that the race was effectively over and that the time had come for Democrats to unite behind Mr. Biden.
“The chance of getting the nomination is so tiny, the stakes are so high given Trump and coronavirus and the economic fallout, that he does need to face that reality,” Robert Reich, a former labor secretary who endorsed Mr. Sanders, said in an interview on Monday.
Mr. Reich said that some onus was on Mr. Biden to demonstrate to Mr. Sanders that he is sufficiently devoted to progressive causes. The former vice president could do that, Mr. Reich said, by naming Mr. Sanders as a top policy adviser to his campaign or by naming Senator Elizabeth Warren of Massachusetts as his running mate.
“It’s the only way he’s going to get the type of enthusiasm he needs,” he said.
With the Democratic primary at an inflection point as Mr. Sanders considers his next steps, Mr. Biden’s campaign has tried to strike a balance between acting presidential, as if he will be the nominee, and not alienating Mr. Sanders and his loyalists. Last Wednesday, hours after Mr. Biden had collected four more victories, his aides seemed reluctant to call on Mr. Sanders to drop out even as they sent out a memo describing Mr. Biden’s lead as nearly insurmountable.
“Should our broad base of support remain — and we have seen no signs that would indicate otherwise — it will be nearly impossible for Sanders to recoup his current delegate disadvantage,” the Biden campaign memo said.
After Mr. Sanders refrained from attacking Mr. Biden at a news conference that afternoon, and instead presented a list of policy priorities, some of Mr. Biden’s supporters hoped the debate would be the next step toward unifying the party around his candidacy. Hours before the matchup, Mr. Biden extended an olive branch to the Sanders camp, embracing a version of a plan Mr. Sanders has championed that calls for tuition-free public college for many students.
So some Biden backers were caught off guard when Mr. Sanders spent much of Sunday’s debate criticizing Mr. Biden’s long record, which revived their broader frustrations with some of Mr. Sanders’s surrogates who have slashed Mr. Biden in more starkly personal terms.
“I didn’t expect some of the sharp elbows we saw from the senator,” said state Representative Malcolm Kenyatta, Democrat of Pennsylvania and a Biden supporter, even as he praised Mr. Sanders’s pledge to support the eventual nominee.
There were more overt indications of displeasure from the Biden camp directly after the debate. Anita Dunn, Mr. Biden’s chief strategist, compared Mr. Sanders to a political demonstrator, telling reporters that the former vice president had spent the debate “graciously dealing with the kind of protester who often shows up at campaign events, on live television.”
Some Sanders advisers are still hoping to eke out victories on Tuesday in Illinois and Arizona but are also cognizant that the outlook looks increasingly challenging. A poll released on Monday showed Mr. Sanders trailing Mr. Biden in Arizona by 20 percentage points. Florida also votes on Tuesday, and aides acknowledge that the state at this point is effectively out of reach.
Beyond that, the crisis surrounding the coronavirus has cast a cloud of uncertainty over the race, scrambling the primary, forcing Mr. Sanders to forgo his signature rallies and making the process even more unpredictable.
Speaking with CNN after the debate, Mr. Sanders openly questioned the wisdom of holding primaries during a pandemic.
“I would hope governors listen to the public health experts and what they are saying is, you just indicated, we don’t want gatherings of more than 50 people,” Mr. Sanders said. “I’m thinking about some of the elderly people sitting behind the desks, registering people, all that stuff. It does not make a lot of sense.”
Establishment Democrats are desperately hoping to avoid a reprise of 2016, when Mr. Sanders battled to the bitter end against Hillary Clinton; his posture enraged some in the party who later blamed him for stoking division among Democrats that led to her loss to Mr. Trump.
There have been more recent signs that he is reluctant to give up without a fight.
Asked recently whether his attacks would weaken Mr. Biden in the general election against Mr. Trump, Mr. Sanders grew exasperated. “I heard that back in 2016,” he told reporters in Phoenix, before asking whether anointing a candidate before the race was over was “really what democracy is about.”
In an enduring sign of strength, his campaign said it raised $2 million on Sunday, and has hit 10 million individual contributions.
Those close to Mr. Sanders say he may want to forge ahead not because he thinks he can still win but to keep his political revolution alive.
“This is about building that movement for change, not just about him,” said Larry Cohen, the chairman of the Sanders-aligned group Our Revolution. “Obviously, he wants to be the president on every level, but it’s much deeper than that.”
Even if Mr. Sanders does decide to soldier on, his campaign will be severely curtailed because of guidelines for containing the coronavirus demand, a new reality that is not lost on many of his advisers.
In recent days, some of Mr. Sanders’s closest advisers, including his campaign manager, Faiz Shakir; Ari Rabin-Havt, a deputy campaign manager; and Mr. Weaver, decamped to Burlington to strategize there. Several of them gathered around a wood-burning stove on Saturday as Mr. Sanders live-streamed a “fireside chat.”
None have said much publicly, but on Monday the campaign sent a fund-raising email noting that “this primary is far from over.”
Nick Corasaniti, Reid J. Epstein and Katie Glueck contributed reporting.