In mid-January, a few weeks before the Iowa caucuses, Bernie Sanders’ pollster offered a stark prognosis for the campaign: Sanders was on track to finish strong in the first three nominating states, but Joe Biden’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 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 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 Sanders and his wife, Jane, consistently expressed reservations about going negative on Biden, preferring to stick with the left-wing policy message they have been pressing for 40 years.

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

Democratic primaries: Who won where? Show all 27 1 /27 Democratic primaries: Who won where? Democratic primaries: Who won where? Arizona: Biden wins Joe Biden: 39 delegates Bernie Sanders: 28 delegates Getty Democratic primaries: Who won where? Alabama: Biden wins Joe Biden: 41 delegates Bernie Sanders: 7 delegates Michael Bloomberg: 1 delegate Reuters Democratic primaries: Who won where? Arkansas: Biden wins Joe Biden: 17 delegates Bernie Sanders: 9 delegates Michael Bloomberg: 5 delegates AFP/Getty Democratic primaries: Who won where? California: Sanders wins Bernie Sanders: 179 delegates Joe Biden: 127 delegates Michael Bloomberg: 15 delegates Elizabeth Warren: 5 delegates AP Democratic primaries: Who won where? Colorado: Sanders wins Bernie Sanders: 20 delegates Joe Biden: 9 delegates Michael Bloomberg: 9 delegates Elizabeth Warren: 1 delegate AFP/Getty Democratic primaries: Who won where? Florida: Biden wins Joe Biden: 152 delegates Bernie Sanders: 53 delegates Getty Democratic primaries: Who won where? Idaho: Biden wins Joe Biden: 11 delegates Bernie Sanders: 9 delegates EPA Democratic primaries: Who won where? Illinois: Biden wins Joe Biden: 95 delegates Bernie Sanders: 59 delegates AFP Democratic primaries: Who won where? Iowa: Buttigieg wins Pete Buttigieg: 14 delegates Bernie Sanders: 12 delegates Elizabeth Warren: 8 delegates Joe Biden: 6 delegates Amy Klobuchar: 1 delegates Reuters Democratic primaries: Who won where? Maine: Biden wins Joe Biden: 11 delegates Bernie Sanders: 9 delegates Elizabeth Warren: 4 delegates AFP/Getty Democratic primaries: Who won where? Massachussetts: Biden wins Joe Biden: 37 delegates Bernie Sanders: 29 delegates Elizabeth Warren: 25 delegates AP Democratic primaries: Who won where? Michigan: Biden wins Joe Biden: 72 delegates Bernie Sanders: 52 delegates Getty Democratic primaries: Who won where? Minnesota: Biden wins Joe Biden: 38 delegates Bernie Sanders: 27 delegates Elizabeth Warren: 10 delegates Reuters Democratic primaries: Who won where? Mississippi: Biden wins Joe Biden: 34 delegates Bernie Sanders: 2 delegates AFP via Getty Images Democratic primaries: Who won where? Missouri: Biden wins Joe Biden: 44 delegates Bernie Sanders: 24 delegates AP Democratic primaries: Who won where? Nevada: Sanders wins Bernie Sanders: 24 delegates Joe Biden: 9 delegates Reuters Democratic primaries: Who won where? New Hampshire: Sanders/Buttigieg tie Bernie Sanders: 9 delegates Pete Buttigieg: 9 delegates Amy Klobuchar: 6 delegates EPA Democratic primaries: Who won where? North Carolina: Biden wins Joe Biden: 66 delegates Bernie Sanders: 36 delegates Elizabeth Warren: 2 delegates Michael Bloomberg: 2 delegates AFP/Getty Democratic primaries: Who won where? North Dakota: Sanders wins Bernie Sanders: 8 delegates Joe Biden: 6 delegates Getty Democratic primaries: Who won where? Oklahoma: Biden wins Joe Biden: 21 delegates Bernie Sanders: 13 delegates Michael Bloomberg: 2 delegates Elizabeth Warren: 1 delegate Reuters Democratic primaries: Who won where? Florida: Biden wins Joe Biden: 152 delegates Bernie Sanders: 53 delegates Getty Democratic primaries: Who won where? Tennessee: Biden wins Joe Biden: 33 delegates Bernie Sanders: 18 delegates Michael Bloomberg: 10 delegates Elizabeth Warren: 1 delegate Reuters Democratic primaries: Who won where? Texas: Biden wins Joe Biden: 111 delegates Bernie Sanders: 102 delegates Michael Bloomberg: 10 delegates Elizabeth Warren: 5 delegates Reuters Democratic primaries: Who won where? Utah: Sanders wins Bernie Sanders: 12 delegates Joe Biden: 2 delegate Michael Bloomberg: 2 delegates AFP/Getty Democratic primaries: Who won where? Vermont: Sanders wins Bernie Sanders: 11 delegates Joe Biden: 5 delegates AP Democratic primaries: Who won where? Virginia: Biden wins Joe Biden: 66 delegates Bernie Sanders: 31 delegates Elizabeth Warren: 2 delegates AFP/Getty Democratic primaries: Who won where? Washington: Biden wins Joe Biden: 46 delegates Bernie Sanders: 43 delegates AP

While Sanders has not ended his bid, he has fallen far behind 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. His efforts to regain traction have faltered in recent weeks as the coronavirus pandemic has frozen the campaign, and perhaps heightened the appeal of Biden’s safe-and-steady image.

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

Sanders had been “on the brink of winning”, 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 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 sceptical Democrats — and sorely vulnerable to an opponent with Biden’s strengths.

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 — Sanders and his wife.

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

There were also serious operational mistakes: in South Carolina, the campaign effectively deputised 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, Sanders often touted his support from some younger African Americans, seemingly missing the bigger picture.

And for all of Tulchin’s alarm in January about South Carolina, on the eve of the primary he was reassuring 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 Sanders and a group of senior aides, Tulchin reminded the senator that their internal polling had him trailing Biden by only 4 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 Sanders to his supporters and limited his ability to build a majority coalition larger than his own progressive movement.

Sanders’ campaign declined to comment for this article.

It was late January when Zephyr Teachout, a liberal law professor allied with Sanders, wrote a column in The Guardian alleging that Biden had “a big corruption problem”. 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 Sanders and Biden seemed to be underway.

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

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

In January, efforts by Turner and others to direct some campaign resources into Super Tuesday states fizzled against opposition from Shakir and others. Shakir was adamant that Sanders’ 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 Elizabeth Warren, with some advisers viewing her as a serious threat that needed to be quashed and others, including Alexandria Ocasio-Cortez, Sanders’ most important endorser, urging the campaign to seek conciliation.

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

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

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

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

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

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

Cusack urged the Sanders campaign to address voters beyond its progressive base, proposing that the senator give a speech at St Francis College in Brooklyn and citing its namesake’s connection to environmentalism and fighting poverty. “His campaign needs to create a unit that is charged with outreach to groups who do NOT identify as progressive, but have strong views that are aligned with his. EXPAND EXPAND EXPAND,” Cusack wrote in an email to top Sanders allies and advisers, obtained by The New York Times.

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 Sanders could attempt to make peace with a Democratic establishment that remained intensely wary of him.

Arriving in Charleston, South Carolina, before the 29 February state primary, 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 an American Civil War analogy to explain the muscle-flexing strategy. Abraham Lincoln did not issue the Emancipation Proclamation, Weaver said, until after Union troops had routed the Confederacy at the bloody battle of Antietam.​

Sanders’ campaign, like much of the political world, had not anticipated Biden’s roaring comeback after South Carolina’s 29 February primary. Indeed, until then, Sanders’ 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 Sanders that he would vanquish 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 Klobuchar and Warren. He won neither.

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

After being routed across the country, 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,” Sanders said. “So that’s not surprising.”