It was a concession speech, of a kind: Addressing reporters on Wednesday, Senator Bernie Sanders bowed not to Joseph R. Biden Jr., who trounced him in a second consecutive week of primary elections, but to the reality of his political predicament. Stripped of his briefly held status as the Democratic front-runner, Mr. Sanders acknowledged that he was “losing” to Mr. Biden but stopped short of accepting defeat.
That careful distinction — between accepting Mr. Biden’s dominance in the race, and yielding to him altogether — now defines Mr. Sanders’s candidacy, and perhaps the Democratic presidential race as a whole.
For in Burlington, Vt., Mr. Sanders was more visibly humbled than at nearly any other point since he emerged as a national figure five years ago, during his insurgent challenge to Hillary Clinton. There was no defiant battle cry on Wednesday, no vow to fight Mr. Biden all the way to the Milwaukee convention in July, no allegation of election-rigging or insinuation of an establishment conspiracy like he memorably made in 2016.
There was, instead, a matter-of-fact assessment of his own diminished position in the race, followed by an equally unsparing recitation of the policy challenges Mr. Sanders intended to put to Mr. Biden in the coming days.
Reaching for a kind of moral victory, Mr. Sanders insisted that his campaign had “won the ideological debate,” but allowed that he had not persuaded most voters that he was the likeliest candidate to defeat President Trump.
Taken together, it was a stark moment of political de-escalation. Some Democratic officials interpreted it as a signal that Mr. Sanders may not be thirsting for a fight to the bitter end, of the kind he waged against Mrs. Clinton in 2016. Even his criticism of Mr. Biden was largely implicit, framed in terms of questions — about health care, climate change, immigration and more — that seemed intended to push Mr. Biden to address the concerns of Mr. Sanders’s political base.
The great question of the Democratic race may now be whether there is anything Mr. Biden can do to assuage those concerns, and how Mr. Sanders might respond if he does.
Representative Peter Welch of Vermont, who is supporting Mr. Sanders, said that the senator sent a clear sign about his political priorities by emphasizing that defeating Mr. Trump was a goal of overriding importance. While Mr. Welch said it was up to Mr. Sanders to decide when his campaign was over, he was already suggesting on Wednesday that Mr. Sanders consider going on a “unity tour” to bring the Democratic Party together.
“At some point, we’re going to have to unify to achieve the overarching goal of defeating Donald Trump,” Mr. Welch said, calling Mr. Sanders a figure with unique potential to achieve that goal: “Bernie, more than any other person in this country, can help Biden be the person who defeats Donald Trump.”
But Mr. Welch and other Democrats cautioned that Mr. Biden should not expect a quick end to Mr. Sanders’s candidacy, or apply too much pressure on Mr. Sanders in the hope of bringing about that outcome. So far, Mr. Biden has shown no inclination to attempt any coercive measures: In his victory speech on Tuesday evening, the former vice president hailed the energy and commitment of Mr. Sanders’s supporters and offered himself to them as an ally, but made no suggestion that Mr. Sanders should stand aside.
By no means is Mr. Sanders giving Mr. Biden a free pass, however. If his tone was more subdued and the content more clinical — like a policy questionnaire distributed to officeseekers — his remarks on Wednesday cataloged considerable skepticism of Mr. Biden and his competing political vision.
“Joe,” he asked, “how are you going to respond to the scientists who tell us we have seven or eight years remaining to transform our energy system?”
“What are you going to do to make sure that all of our people can go to college or trade school regardless of their income?”
“What are you going to do about the millions of people who are struggling with outrageous levels of student debt?”
Still, after Mr. Biden’s string of wins on Tuesday night, both camps seem to recognize that the two men will most likely need to find ways to bring Democrats together behind Mr. Biden this fall. It is a dual challenge that does not play to either man’s strengths: convincing a zealous collection of young and progressive voters that Mr. Biden — an institutionalist moderate who speaks of record players and long-dead Senate colleagues — is the man to meet the moment.
For Mr. Sanders, who has spent the past year suggesting that any candidate besides himself would be insufficiently bold for the times, an eventual endorsement for Mr. Biden would not necessarily persuade admirers that an establishment Democrat should be trusted. It might take a more robust show of support from Mr. Sanders, and even then there is no guarantee that the affections of Mr. Sanders’s base can be transferred.
For Mr. Biden, who has antagonized Mr. Sanders as a non-Democrat out to diminish the presidency of Barack Obama, even a firm insistence that he understands what is driving Sanders voters will be viewed skeptically.
But despite Mr. Biden’s success during the last two weeks of primaries, he will need to make gains with some key Sanders constituencies — not only younger liberals but also Latinos across several age brackets — to build the most formidable coalition possible in the fall. And he will almost certainly need Mr. Sanders’s help to get there.
Some veterans of Mrs. Clinton’s campaign would advise Mr. Biden not to expect much. Though Mr. Sanders officially endorsed Mrs. Clinton in 2016 and rallied for her, her aides generally viewed the endorsement as somewhere between dutiful and inadequate — especially when weighed against the months of negative attacks from Mr. Sanders and his supporters depicting her as a corporate tool well after she appeared to have the nomination in hand.
There is some reason to believe that this election might be different. Mr. Sanders appears to harbor much less visceral disdain for his opponent than he did in 2016, when he and his aides seethed constantly at the perceived coronation of Mrs. Clinton. Mr. Sanders has never attacked Mr. Biden’s character or his personal ethics, even as a good number of his political allies have done so repeatedly.
“It needs to be a two-way dance,” said Brian Fallon, Mrs. Clinton’s national press secretary four years ago, noting that even a progressive like Senator Elizabeth Warren had been found wanting by Sanders fans. “What the courting looks like, what will actually satisfy large chunks of his movement, is a mystery. So much of that movement could not even find Warren as an acceptable alternative. So, what the heck can Joe Biden do?”
Mr. Fallon did have a few ideas: namely, some policy drift toward Mr. Sanders’s platform on climate or health care issues from the Biden camp if Mr. Sanders leaves the race.
At the moment, the Sanders base appears in no mood to settle. Some of his top surrogates continue to tweet eagerly amid his defeats on Tuesday night about Mr. Biden’s weaknesses and elevate any scrap of good news about Mr. Sanders, from his strength with young voters to his projected victory in North Dakota.
“Nah, it’s not over,” wrote Shaun King, a civil rights activist and Sanders surrogate with over a million followers on Twitter.
“Not over indeed,” said Nina Turner, a former Ohio state senator and national campaign co-chair for Mr. Sanders.
“We are just getting started,” tweeted Representative Rashida Tlaib of Michigan, one of the senator’s highest-profile endorsers.
But there was also an awareness among Sanders allies that he has more at stake in this campaign than in 2016, and that the way his candidacy ends — if indeed it ends in defeat — could have far-reaching implications for the fate of the agenda he has championed and the movement he has built.
“Whatever he does, he’s going to go forward with a base of support that’s going to continue to fight for issues,” said James J. Zogby, a longtime Sanders ally who is the president of the Arab American Institute, in an interview on Tuesday night.
Mr. Sanders, he said, had a longer legacy than a presidential campaign: “He is a transformative figure in our political life.”
It falls now to Mr. Sanders to square his grand stature, as an individual leader, with the perilous state of his presidential campaign. Mr. Welch, who has risen in Vermont politics alongside Mr. Sanders for more than three decades, urged his colleague not to bank on an astounding turn of fortune of the kind Mr. Biden enjoyed after the South Carolina primary.
“We basically had a miracle in this election a couple weeks ago,” Mr. Welch said. “And Joe Biden was the beneficiary of that miracle.”
For Mr. Sanders to expect another, he added, would be “asking a lot of the higher powers.”