“It was this scorched-earth approach,” said Markos Moulitsas, founder of the pioneering liberal website Daily Kos, who had supported Sen. Elizabeth Warren (D-Mass.). “There was no opening for common ground.”

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Rep. Alexandria Ocasio-Cortez (D-N.Y.), a prominent Sanders supporter who has campaigned with him, took issue with Sanders backers who have been attacking Warren. Warren’s decision not to endorse Sanders after she exited the race, while criticizing Sanders for his allies’ online taunts, was a notable rebuke from an ideological ally.

“When things start falling short, I don’t think seeking out who to blame, instead of identifying how to adapt, is the smart thing to do,” Ocasio-Cortez said in an interview. “I think we have to identify, ‘Okay, how do we bring in people?’ It’s like when I ran in my primary, I didn’t win only with supporters of Senator Sanders. We built a broad coalition on the same principles.”

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Some in the Sanders campaign have adopted a similar credo. Two top campaign officials spent hours working the phones late Saturday night to finalize a coveted new endorsement from civil rights icon Jesse Jackson. But on Tuesday night, it was clear that Sanders’s overarching approach was in trouble. Punctuating the week since Super Tuesday, Biden scored resounding wins in Michigan and three other states, moving closer to a daunting lead in delegates.

Sanders’s losses a week earlier had created a moment of reckoning for a campaign that was well-suited for a fractured field but ill-prepared for a swift consolidation by his opponents. Sanders campaign officials for months had talked up the power of their limited but passionate following in a crowded race where the opposition was split.

When the party rallied around Biden almost overnight, it was a gut punch and the campaign struggled to recover. Sanders allies were especially stunned by how fast Pete Buttigieg and Sen. Amy Klobuchar, two prominent centrists, dropped out and endorsed Biden.

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“You basically had the most unprecedented event happening in the history of presidential primaries,” said Sanders pollster Ben Tulchin. “Coming out of that unprecedented event, we had to retool and focus on the March 10 and March 17 states.”

The campaign threw all it had into winning Michigan, a delegate-rich state where Sanders landed his biggest upset four years ago against Hillary Clinton.

If Sanders could show he was stronger than Biden in a state that was key to President Trump’s victory, the campaign would be back, officials believed. If not, they recognized, Sanders would face the kind of serious trouble he found himself in Tuesday night.

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When the campaign scrapped plans for Sanders to spend time in Mississippi in favor of Michigan, it signaled to many Democrats that he was effectively giving up on black voters in the South, who have strongly favored Biden.

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At the same time, campaign officials drew up a plan for Sanders to step up his attacks on the former vice president, sending a memo to supporters even before Super Tuesday declaring a new phase in the fight and releasing a TV ad that attacked Biden by name for the first time.

As the former vice president racked up new endorsements each day, portraying himself as a unifying force and the increasingly inevitable nominee, Sanders, who associates say is less comfortable directly attacking Biden or anyone else than talking up his own record, opened up new fronts against his chief rival, seeking to undercut him on gay rights and abortion rights.

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At a rally Monday in St. Louis, Sanders ticked off a litany of issues on which he said Biden had been wrong. “Joe Biden voted for the war in Iraq. I voted against the war in Iraq,” he said. “Joe Biden voted for the Wall Street bailout. I helped lead the effort against the Wall Street bailout. Joe voted for disastrous trade agreements. . . . I helped lead the effort against those terrible trade agreements.”

Then he took aim at one of Biden’s chief selling points — electability. “My point here is to ask you to think that in a general election, which candidate can generate the enthusiasm and the excitement?” Sanders said.

But while Sanders made this relatively straightforward case, some supporters took to social media to lob attacks that struck some Democrats as uglier, going after Biden’s cognitive abilities. Among them was Shaun King, a prominent Black Lives Matter activist who has introduced Sanders at some events.

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“It’s genuinely abusive at this point to hand this man a microphone and ask him to interact with people,” King wrote Sunday on Twitter, retweeting a message alleging that Biden’s “brain stops working for a few seconds” in a video clip. “It’s painful to watch.”

In an interview, King defended his critique, arguing that primaries ought to put candidates through a “gantlet of examinations” and assess how they would fare in debates against Trump.

“I don’t approve of people making armchair medical diagnoses, but like, I do see Biden and he is a diminished version of his best self,” said King. “It’s kind of the elephant in the room.”

That line of attack echoed shots at Biden taken by the Trump camp, a point not lost on some other Democrats. And it met resistance from other Sanders supporters who preferred that the contrasts remain in the realm of policy.

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Biden, who has often stumbled over his words, released a letter from his doctor last year describing him as a “healthy, vigorous 77-year-old male.” He has also opened up about overcoming a stutter.

“I’ve made it very clear that I don’t think trying to make these health arguments is fair,” said Abdul El-Sayed, a 2018 candidate for governor of Michigan who campaigned with Sanders on Monday. “We’ve got to focus on the issues.”

Other Sanders supporters were also focusing on trying to win new allies, with Rep. Ro Khanna (D-Calif.) at the forefront of the effort. In addition to going out of his way to praise the party publicly, Khanna worked with Nina Turner, his fellow Sanders campaign co-chair, to finalize the Jackson endorsement, according to people with knowledge of the situation, who spoke on the condition of anonymity to be candid about the internal discussions.

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The duo spent several hours speaking with Jackson and his inner circle Saturday night, following up on a call Jackson made call to Turner on March 3. They helped finalize commitments on the Sanders campaign’s part to address issues important to Jackson, the people said. When the campaign announced the endorsement and the pledges Sunday, Sanders called it “one of the honors of my life.”

It was a significant win, given Jackson’s decades-long prominence as a civil rights leader. Yet Sanders’s efforts to improve his standing among black voters have been awkward in recent days. As Khanna and Turner worked to finalize the Jackson endorsement, Sanders appeared in Flint, Mich., at an event billed as a town hall on racial justice.

Yet of the 1,200 attendees, only about three dozen were black. Then the senator decided at the last minute not to deliver his planned speech contrasting his record with Biden’s on racial justice issues — because, a spokesman said, he wanted to let the African American panelists onstage speak about their own experiences.

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Still, the effect was to suggest that Sanders continues to be uncomfortable delving personally into issues that affect black people’s lives.

Alongside these fumbling efforts to court black voters, Sanders was lumping many who opposed him into his derision toward the “establishment.” After Super Tuesday, some felt that included millions of Democratic voters, as well as Buttigieg, the onetime mayor of South Bend, Ind., and Klobuchar, a senator from Minnesota.

“One of the things that I was kind of not surprised by is the power of the establishment to force Amy Klobuchar, who had worked so hard, Pete Buttigieg, who, you know, really worked extremely hard as well, out of the race,” Sanders said Sunday on ABC’s “This Week.” He said the Democratic establishment wanted to ensure “that people coalesced around Biden.”

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That prompted a stern reply from one of Buttigieg’s top campaign aides, Lis Smith, who wrote on Twitter that the former mayor’s decision “was his and his alone.”

But speaking on NBC the same day, Sanders said that had Buttigieg and Klobuchar not dropped out, he would have won Massachusetts, Minnesota and Maine — all states where Biden prevailed last week. It was his most direct acknowledgment of the campaign’s long-held belief that a fractured Democratic field would deliver him victory.

Instead, the losses in those states had a chilling effect on some Sanders advisers, who were privately alarmed that Biden won in places where he did not even campaign.

The sense of fatalism grew Tuesday night. Ocasio-Cortez, speaking on Instagram Live after the results began coming in, enumerated the left’s recent wins and appeared to suggest a landscape beyond Sanders. “Now is the time to leverage that,” she said. “Now is the time to ask for accountability. And now is the time to ask for commitments — real commitments — not just gestures.”

Sanders’s inability to grow his coalition at this moment was especially damaging given that a central premise of his candidacy is that he will expand the electorate, attracting young people and others new to politics. That, in theory, would compensate for his challenge in attracting more traditional Democrats.

But while Biden has rounded up support from former candidates from the party’s centrist wing, Sanders has been unable thus far to win the backing of Warren, a fellow liberal and a prominent figure in the Democratic Party.

Relations between the two were strained earlier this year by a dispute about a private conversation before the campaign began, when Warren contends Sanders told her a woman could not win the presidency, something he sharply denies saying.

Instead, Sanders sought to rely on prominent liberals who are already in his corner to help him turn things around, for example campaigning Sunday with ­Ocasio-Cortez in Ann Arbor, Mich.

On the trail and in an interview, Ocasio-Cortez struck a more inclusive chord than many other Sanders supporters, talking about the need to build a broad liberal movement that goes beyond individual candidates.

“We have to do that from a fundamentally inclusive and positive and welcoming gesture, a welcoming kind of position,” ­Ocasio-Cortez said. Declining to urge Warren to endorse Sanders, she said, “I don’t think it’s about telling anybody what to do. I think it’s about, how do we invite people, and how do we make our movement bigger, and how do we adapt it to be as inclusive a possible while retaining our values.”

The past week showcased how much the Sanders campaign is an unruly coalition rather than a tight operation, as allies voiced various conflicting theories about what he needed to do.

Some said Sanders needs to show a more personal side, a recurring suggestion that Sanders has repeatedly been reluctant or unable to embrace.

“He has got to really speak from the heart to people,” said RoseAnn DeMoro, a close friend of Sanders who emailed the senator and his wife last week encouraging this more personal pitch. “We’re going up against sharks,” she added, and Sanders should “tell America who he is.”

The campaign has also experienced a push-and-pull over strategic decisions, debating in recent days whether to release internal polling showing a competitive race with Biden in Michigan, for example, according to people with knowledge of the situation. In the end, the campaign did not release any polling data, opting not to risk setting expectations and then falling short.

By the end of Tuesday, Sanders had been dealt a significant loss in Michigan, leaving his path forward more uncertain than ever.