Bernie Sanders' endorsement of Joe Biden in the Democratic presidential race Monday represented the official end of his effort to reshape the party in his own image. But it was March 4, the Wednesday after a not-so-Super Tuesday when the Vermont senator's team knew his quest for the nomination was all but over.

For months, Sanders' team had signaled a desire to have a one-on-one battle with Biden: a proxy for the yearslong ideological clash.

What they didn't expect was that it would come over a period of hours just ahead of Super Tuesday, in a coordinated and ultimately successful effort to stop Sanders. One after another, the other candidates exited the race and threw their support behind Biden.

In interviews with NBC News, Sanders aides reflected on his campaign with a mixture of frustration, sadness and confusion — believing some decisions and inadequate foresight led to the bleak position the candidate faced after Super Tuesday.

A failure to capitalize on his brief front-runner status, they said, combined with a strategy built on the support of young voters and a reliance on the continued presence of a large Democratic field proved a costly miscalculation that thwarted his candidacy with shocking efficiency.

The path collapses

Sanders entered the 2020 race in an enviable position, with national name recognition and a sturdy base of support from his 2016 campaign against Hillary Clinton. And while that loyal group of progressives and young people would carry him to a popular vote win in Iowa, an outright win in New Hampshire and a decisive victory in Nevada, he was not winning a majority of the votes in any of those contests and was benefiting from a multicandidate field.

And there was no sign he was expanding his support, particularly among African Americans.

"There was no ability or desire to grow the electorate," a former Sanders campaign official said, requesting anonymity in order to speak freely.

Senior campaign aides acknowledged their initial strategy had rested on the candidate eking out narrow wins over a period of months against a broad field of competitors. The early state success, they said, would create a path to earning a majority of delegates.

"What was clear is that campaign leadership was banking on a brokered convention," the former official continued — suggesting Sanders would arrive at the Democratic gathering in Milwaukee short of the 1991 delegates needed, but with a strong enough plurality that the party would have no choice but to cede him the nomination.

Then Biden came back from near-elimination, cruising to a blowout victory in South Carolina, aided by strong support from its heavily black Democratic electorate. Rivals Pete Buttigieg, the former mayor of South Bend, Indiana, and Minnesota Sen. Amy Klobuchar quickly dropped out to endorse him, and the momentum Sanders had been riding all but disappeared.

Billionaire Mike Bloomberg quickly exited the race after Super Tuesday, endorsing Biden, and Massachusetts Sen. Elizabeth Warren would drop out days later.

The uncomfortable front-runner

Some advisers said Sanders erred by not making more of an effort to welcome skeptics into his movement after the Nevada win made him the acknowledged front-runner. Indeed, he seemed to revel in conflict, continuing to deride what he called the "Democratic establishment," even as he was seeking to be that party's standard-bearer.

"They're getting nervous!" was the common refrain that he would repeat at rallies to thunderous cheers.

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Sanders' rejection of the broader party — and the problems that caused — in many ways, mirrored what his supporters say they love about him the most: he's not a normal politician.

He once chided a baby for making noise during a town hall. He told The New York Times editorial board, he doesn't "tolerate bullshit" terribly well. During a CNN town hall in February, Sanders was asked if he considers himself the Democratic front-runner and he responded, "Who cares?"

Those rough edges may have endeared him to die-hard fans but ultimately proved problematic in wooing converts. Aides admit it was a struggle to get Sanders on the phone to "play politics" and work to secure influential endorsements.

"The moment he understands this is a relationship-building call, and not something where he's immediately accepting the endorsement, he says 'Oh, come on, don't waste my time' and hangs up and keeps moving," a Sanders aide said.

That unwillingness to nurture relationships proved costly. Sanders aides said that while working to close the gap with Biden in South Carolina, their efforts were stymied when the state's most influential Democrat, Rep. Jim Clyburn, announced his endorsement of Biden. Clyburn later said Sanders had never called for his support.

Banking on young progressives

In early October, Sanders walked out of the Desert Springs Hospital in Las Vegas with his fist in the air. He had suffered a heart attack four days prior, and many in the political and pundit class had declared his campaign over. But he was holding onto a powerful endorsement that had the potential to revive his campaign.

Two weeks later, in front of 26,000 people in New York's Queensbridge Park, Sanders stood on stage with New York Rep. Alexandria Ocasio-Cortez, arguably the nation's most popular young Democratic elected official.

"I am back," Sanders said after accepting the coveted endorsement from "AOC," as she is known. Her support helped convince progressives that he wasn't going anywhere.

Throughout the campaign and into its final days, Sanders was eager to point to the support he received from young leaders and voters.

Senior officials acknowledged Sanders was never going to be able to win a nomination with the youth vote alone, especially among traditional primary voters. But they did expect a surge in turnout of those 45 years and younger that never materialized. Instead, state by state, exit polling repeatedly showed younger voters were a smaller share of the electorate, and his margins among this group in many Super Tuesday states did not even match his 2016 performance.

When he announced last week he would suspend his campaign, Sanders pointed to the youth vote as part of his legacy.

"We received a significant majority of the votes, sometimes overwhelming majority from people not only 30 years of age or under, but 50 years of age or younger. In other words, the future of this country is with our ideas," Sanders said.

Last stand

"We needed something to change the narrative," one aide said, explaining the campaign's thinking after the tide began turning against Sanders.

Many in Sanders' orbit blamed the media for his overall struggles, but said the coverage of Biden's Super Tuesday-eve endorsements was an uncontrollable "structural force" that had more of an impact than any late campaign decisions they could have made.

"The consolidation of moderates occurred so quickly and swiftly, and produced a ton of positive media coverage for Joe Biden and negative media coverage for Bernie," a senior aide said.

Acknowledging Sanders' unrelenting and ultimately losing struggle to earn the support of black voters, the campaign made a calculation: the candidate would shore up and turn out existing supporters rather than persuade skeptics to give him another look.

The day after the South Carolina primary illustrated that choice. Choosing between the commemoration of Bloody Sunday, a key moment in civil rights history, in Selma, Alabama, or a rally of his own in Los Angeles, the Sanders campaign chose the latter. Later, deciding between a rally with the influential mayor of Jackson, Mississippi, or local media interviews in Michigan, Sanders stuck with Michigan.

Advisers believed that Michigan, where Sanders had notched a surprise win over Clinton in 2016, was their last best hope to rescue the campaign post-Super Tuesday. Early state staffers and surrogates had redeployed there the week leading up to the March 10 primary, with staff from the headquarters quickly following.

In the end, Sanders lost Michigan by more than 200,000 votes, not winning a single county.

"If you looked at the basic strategic decisions that we made, I rest comfortably saying we put our best foot forward, made the best calls given our candidate, and it just wasn't meant to be," campaign manager Faiz Shakir said.

The coronavirus takes hold

Hopes to mount a comeback were ultimately blocked by the coronavirus pandemic.

At a time when supporters argued the candidate's platform was more relevant than ever, Sanders was mostly confined to his home. There were no in-person rallies and no chance to interact with voters while highlighting the need for universal health care and his signature plan, "Medicare For All."

Sanders did quickly pivot his campaign infrastructure, holding 10 coronavirus-focused livestreams that attracted millions of overall views. But with a health and economic crisis sweeping the nation, Sanders was unable to alter the narrative that he was losing.

The night after he suspended the campaign, he sat for an interview with MSNBC's Chris Hayes. Noting the Sanders campaign had pledged to cover the health care costs for all staffers through the end of October, Hayes told Sanders the gesture was "deeply decent."

When the campaign's Digital Video Producer Chris Witschy thanked the senator for that gesture after cameras were off, Witschy said Sanders responded, "Don't worry about it. It's the right thing to do and people have enough to deal with these days."

To Sanders, the "struggle" continues.