In mid-January, a few weeks before the Iowa caucuses, Senator Bernie Sanders’s pollster offered a stark prognosis for the campaign: Mr. Sanders was on track to finish strong in the first three nominating states, but Joseph R. Biden Jr.’s powerful support from older African-Americans could make him a resilient foe in South Carolina and beyond.

The pollster, Ben Tulchin, in a meeting with campaign aides, recommended a new offensive to influence older black voters, according to three people briefed on his presentation. The data showed two clear vulnerabilities for Mr. Biden: his past support for overhauling Social Security, and his authorship of a punitive criminal justice law in the 1990s.

But the suggestion met with resistance. Some senior advisers argued that it wasn’t worth diverting resources from Iowa and New Hampshire, people familiar with the campaign’s deliberations said. Others pressed Mr. Tulchin on what kind of message, exactly, would make voters rethink their support for the most loyal ally of the first black president.

Crucially, both Mr. Sanders and his wife, Jane, consistently expressed reservations about going negative on Mr. Biden, preferring to stick with the left-wing policy message they have been pressing for 40 years.

The warnings about Mr. Biden proved prescient: Two months later, Mr. Sanders is now all but vanquished in the Democratic presidential race, after Mr. Biden resurrected his campaign in South Carolina and built an overwhelming coalition of black voters and white moderates on Super Tuesday.

While Mr. Sanders has not ended his bid, he has fallen far behind Mr. Biden in the delegate count and has taken to trumpeting his success in the battle of ideas rather than arguing that he still has a path to the nomination.

In the view of some Sanders advisers, the candidate’s abrupt decline was a result of unforeseeable and highly unlikely events — most of all, the sudden withdrawal of two major candidates, Senator Amy Klobuchar and former Mayor Pete Buttigieg, who instantly threw their support to Mr. Biden and helped spur a rapid coalescing of moderate support behind his campaign.

Mr. Sanders had been “on the brink of winning,” Mr. Tulchin argued, “until the most unprecedented event in the history of presidential primaries occurred.”

But interviews with more than three dozen Sanders aides, elected officials, activists and other people who worked with his campaign revealed a more extensive picture of his reversal of political fortune. Though Mr. Sanders climbed to a position of seeming dominance by mid-February, he and his inner circle also made a series of fateful decisions that left him ill positioned to win over skeptical Democrats — and sorely vulnerable to an opponent with Mr. Biden’s strengths.

Mr. Sanders proved unable to expand his base well beyond the left or to win over African-Americans in meaningful numbers. He failed to heed warnings from traditional party leaders, and even from within his campaign, about the need to modulate his message and unify Democrats. He allowed internal arguments to fester within his campaign, an ungainly operation that fragmented into factions beneath the only two real decision makers — Mr. Sanders and his wife, Jane.

Though outwardly amiable, Mr. Sanders’s inner circle fractured between some long-serving counselors and relative newcomers, like Faiz Shakir, his campaign manager. Mr. Shakir and others regarded pleas from Mr. Tulchin and another pugilistic aide, David Sirota, to go on the attack against Mr. Biden as both futile and annoyingly predictable, while Mr. Shakir’s internal critics saw him as exceedingly territorial.

There were also serious operational mistakes: In South Carolina, the campaign effectively deputized a former Ohio state senator and loyal surrogate, Nina Turner, to direct strategy, rather than empowering a political strategist to run the pivotal early state. In private conversations, Mr. Sanders often touted his support from some younger African-Americans, seemingly missing the bigger picture.

And for all of Mr. Tulchin’s alarm in January about South Carolina, on the eve of the primary he was reassuring Mr. Sanders that a public poll showing him down over 20 percentage points in the state was “an outlier for good reason.”

In an email sent to Mr. Sanders and a group of senior aides, Mr. Tulchin reminded the senator that their internal polling had him trailing Mr. Biden by only four points. Two days later, the former vice president would win South Carolina by nearly 30 points.

Perhaps the most significant factor, as with every presidential campaign, was the candidate himself, and the stubborn ideological and stylistic consistency that both endeared Mr. Sanders to his supporters and limited his ability to build a majority coalition larger than his own progressive movement.

Mr. Sanders’s campaign declined to comment for this article.

It was Mr. Sanders’s persistent lashing of the “political establishment” that concerned Representative Peter Welch, a liberal Democrat and fellow Vermonter who was one of just a few members of Congress to endorse Mr. Sanders’s campaign.

Mr. Welch said he had reached out to the campaign last month to implore Mr. Sanders to ease up on that rhetoric, which Mr. Welch believed sounded exclusionary to ordinary people backing other candidates. After all, Mr. Welch said, there were “a lot of voters who are just everyday voters, who decided to vote for other Democrats.”

An Inner Circle Divided

It was late January when Zephyr Teachout, a liberal law professor allied with Mr. Sanders, wrote a column in The Guardian alleging that Mr. Biden had “a big corruption problem.” Mr. Sirota, the Sanders aide, who is known for his voluble and combative online persona, quickly blasted out her column to his large email list. A new phase of conflict between Mr. Sanders and Mr. Biden seemed to be underway.

But Mr. Sanders put a stop to it. “It is absolutely not my view that Joe is corrupt in any way,” Mr. Sanders told CBS News.

In private, Mr. Sanders’s campaign went further, according to two people familiar with the internal turmoil. As punishment for stirring the controversy, Mr. Sirota, who is based in Colorado, was barred from traveling for the campaign outside of visits to its Washington headquarters.

The conflict over Ms. Teachout’s column was part of a long-running debate within the Sanders campaign about what approach to take with Mr. Biden. A small group of advisers — including Mr. Tulchin, Ms. Turner and Mr. Sirota — regularly pleaded with Mr. Sanders to attack the former vice president.

But Mr. Sanders resisted, giving speech after speech scorching unnamed establishment Democrats but declining to pursue Mr. Biden directly. He ruled out several lines of attack against the former vice president because they touched on Mr. Biden’s role in the Obama administration, which Democratic primary voters revere.

Mr. Shakir and a second senior aide, Ari Rabin-Havt, took Mr. Sanders’s side and repeatedly reminded other campaign officials that Mr. Sanders was the ultimate decision maker on the campaign. In conversations with associates, both men agreed that it might make sense to criticize Mr. Biden in a sharper way. But they said Mr. Sanders could not be persuaded to do so: He and Jane liked the Bidens personally, and their word was final.

The fissures within the campaign leadership extended beyond how to deal with Mr. Biden.

In January, efforts by Ms. Turner and others to direct some campaign resources into Super Tuesday states fizzled against opposition from Mr. Shakir and others. Mr. Shakir was adamant that Mr. Sanders’s path to the nomination ran principally through Iowa, New Hampshire and Nevada, and the California primary on Super Tuesday.

There was also a running argument within the campaign about how to handle Senator Elizabeth Warren, with some advisers viewing her as a serious threat that needed to be quashed and others, including Representative Alexandria Ocasio-Cortez, Mr. Sanders’s most important endorser, urging the campaign to seek conciliation.

The dispute erupted publicly in January, when CNN reported that Mr. Sanders had told Ms. Warren in 2018 that he did not believe a woman could defeat President Trump, an assertion Mr. Sanders denied.

Mr. Shakir escalated the conflict, daring Ms. Warren on TV to call the report a “lie.” Jeff Weaver, a top Sanders adviser, took a different approach in his own TV appearance, suggesting there had been a misunderstanding — a step he told associates was aimed at calming things down.

But Ms. Warren stood by the account, and a clash between her and Mr. Sanders consumed the Democratic debate in Iowa. It would linger over both of them for the remainder of the race.

Revolutionary to a Fault

Despite the divisions within his campaign, Mr. Sanders cut a winning path through the first few states to vote, culminating with a landslide victory in Nevada on Feb. 22. In his speech that night, Mr. Sanders sounded a unifying note, focusing on his “multigenerational, multiracial coalition.”

Encountering a pair of reporters in a Las Vegas hotel that evening, Mr. Tulchin — strolling to dinner with Mr. Weaver — crowed that Mr. Sanders had delivered a speech worthy of the general election. Mr. Weaver was more subdued, noting that the primary fight was not over.

The speech turned out to be a blip between Mr. Sanders’s anti-establishment diatribes. And there was little aides could do to steer him in a different direction: The chief speechwriter on the Sanders campaign was Mr. Sanders.

For months, his political advisers and outside allies had quietly mulled a shift in tone — the possibility that Mr. Sanders might take even modest steps to show skeptical Democrats that he could unify the party.

But he has always been disdainful of the art of politics and had to be nudged into wooing even friendly Democratic leaders. As Ms. Warren relentlessly courted Ms. Ocasio-Cortez last fall, Ms. Ocasio-Cortez’s advisers had to prod Mr. Sanders’s aides into having him call her — a conversation that eventually led to her endorsing him.

Pushing Mr. Sanders to reach out to “establishment Democrats” whom he regularly taunted was even tougher — despite the best efforts of even some of his staunchest supporters on the left. Ms. Ocasio-Cortez repeatedly urged the campaign to broaden Mr. Sanders’s message and seek out new allies, outside his familiar base. (In a statement, Ms. Ocasio-Cortez denied any “tension or major disagreements” with Mr. Sanders.)

RoseAnn DeMoro, the former leader of the nurses union who was one of Mr. Sanders’s most ferocious surrogates in 2016, and the actor John Cusack, another ally, both pressed the campaign to refocus Mr. Sanders’s pitch on a general-election audience, people familiar with their entreaties said.

Mr. Sanders was not interested in moving in that direction. Some advisers, who endured the divisive 2016 campaign, believed that it was only after seizing a dominant advantage that Mr. Sanders could attempt to make peace with a Democratic establishment that remained intensely wary of him.

Arriving in Charleston, S.C., ahead of the Feb. 29 state primary, Mr. Weaver said the campaign had not yet sought a working relationship with figures like the House Speaker Nancy Pelosi because they wanted first to demonstrate the full sweep of their coalition on Super Tuesday three days later. He reached for a Civil War analogy to explain the muscle-flexing strategy. Abraham Lincoln did not issue the Emancipation Proclamation, Mr. Weaver said, until after Union troops had routed the Confederacy at the bloody battle of Antietam.

In the weeks before Super Tuesday, Mr. Sanders had indeed refused to yield to critics who were searching for gestures of accommodation. In Nevada, leaders of the powerful Culinary Workers Union Local 226, were frustrated that Mr. Sanders did not speak out more forcefully when his supporters heaped abuse on predominantly female union leaders over their opposition to “Medicare for all.”

As Culinary officials were deluged with vitriol, including graphic and misogynistic messages, Mr. Sanders placed a phone call to the union’s secretary-treasurer, Geoconda Argüello-Kline, but they never connected.

“Our feeling was that Senator Sanders should have said something earlier and should have been stronger about it,” said Bethany Khan, a political strategist for the union.

And then there was Cuba: In February, Democratic officials in Florida reached out to propose convening some conversations between Mr. Sanders and Latino leaders in South Florida, many of whom were nervous about his rhetoric on socialism and revolution. The Sanders campaign showed tentative interest, people familiar with the conversations said.

Then, the day after the Nevada caucuses, CBS aired a “60 Minutes” interview in which Mr. Sanders stood by his past praise for certain policies of Fidel Castro’s government. Democrats across Florida, including prominent members of Congress, erupted in dismay.

Mr. Sanders stood his ground, including in a televised debate. And he made no attempt at private damage control: None of the lawmakers who rebuked him in public heard from Mr. Sanders or his senior aides.

His campaign eventually dispatched emissaries, including the actress and activist Cynthia Nixon, to meet quietly with Latino groups in Orlando after Super Tuesday, but they could not assure local Democrats that Mr. Sanders would change course before Florida’s March 17 primary.

“It may be authentic to Bernie, but it does not work in Florida,” said Alex Barrio, a Democratic activist in Florida who attended the meeting. “It’s conceding Florida in a monumental way.”

By the time Florida voted, however, the race was all but over.

Mr. Sanders’s campaign, like much of the political world, had not anticipated Mr. Biden’s roaring comeback after South Carolina’s Feb. 29 primary. Indeed, until then, Mr. Sanders’s campaign was expecting to win seven or eight of the 14 states voting on Super Tuesday and seize a solid delegate lead over the rest of the Democratic field.

So confident was Mr. Sanders that he would vanquish Mr. Biden that he spent valuable days trying to force two other candidates out of the race by campaigning in Minnesota and Massachusetts, the home states of Ms. Klobuchar and Ms. Warren. He won neither.

Mr. Sanders had suddenly become a spectator in the campaign, powerless to stop a tectonic shift against him by the party’s moderate wing. Ms. Klobuchar called Mr. Sanders before announcing her endorsement of Mr. Biden, while Mr. Sanders and Mr. Buttigieg did not speak.

After being routed across the country, Mr. Sanders knew who to blame in an appearance on ABC’s “This Week.”

“What the establishment wanted was to make sure that people coalesced around Biden and try to defeat me,” Mr. Sanders said. “So that’s not surprising.”

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