WASHINGTON — Three years ago, Bernie Sanders met with a group of allies on Capitol Hill on a spring day to discuss a potential second bid for the White House.

One adviser argued that the Vermont independent senator would need to win a few more Democratic endorsements if he ran again — just enough to unravel the barbarian-at-the-gates perception — and urged Sanders to consider what a 2020 bid would look like with the backing of a political heavyweight like Rep. Jim Clyburn, the top Democrat in South Carolina.

It’s not a question Sanders wanted answered: the senator never sought the leading black lawmaker's support and Clyburn endorsed Joe Biden just when the former vice president's bid appeared to stall. An overwhelming win in Clyburn's state at the end of February turned into a dominant Super Tuesday showing that allowed Biden to begin amassing the kind of delegate lead that Sanders, when he dropped out of the race for the nomination Wednesday, acknowledged he could not overcome.

The unsought endorsement underscored a campaign strategy that would eventually prove fatal. The senator and his team bet big on a small-tent strategy: one that spent little effort broadening the base or building a wider coalition, but instead leaned into Sanders' existing supporters in the hopes that a divided presidential primary field would open up a path for him to win the nomination without having to dramatically expand his base.

“Fair or unfair, Bernie Sanders carried a lot of scars into this campaign,” Mark Longabaugh, a former top adviser who worked on the senator’s first bid, said. “Those scars made it hard for him to add on new constituencies that hadn’t been part of his constituency last time.”

Surveying the 2020 landscape, Sanders advisers accurately foresaw a crowded Democratic primary contest and counted on the field remaining fractured so his committed minority of supporters might be enough for him to form a plurality and win the day. Sanders allies had cut superdelegates out of the nominating process by pressuring the Democratic National Committee to change its rules, clearing an obstacle for someone like him to earn the nomination on the first ballot at the convention without the establishment’s support.

That strategy counted on him consistently winning 30-40 percent of Democratic voters, as Donald Trump had done with the Republicans in 2016, and it counted on the field staying divided among numerous candidates.

But his early successes in Iowa and New Hampshire revealed a fundamental problem: His support base was actually closer to a quarter of the party rather than a third of it, and that was in states that would be among the most demographically favorable to him.

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“You can’t win plurality victories with 25 percent of the vote, you have to get up 40, 45 percent,” Longabaugh said.

The field also didn't remain fractured forever: mainstream candidates dropped out and united behind Biden, making it another head-to-head race between Sanders and an establishment Democratic front-runner.

Comparisons between Sanders’ performance in 2016 to 2020 in key states like Florida showed he was underperforming the second time around, suggesting many of his votes four years ago were more about opposition to Hillary Clinton than support for him.

While many Democrats expected Sanders would devote his second presidential bid to trying to win over people who weren’t with him in 2016, or at least making skeptics feel more comfortable with the idea of him leading the party, the candidate and his supporters instead focused on highlighting the line between him and the rest of the party, dwelling on how he was right while others were wrong last time and continuing his call for a “political revolution” to overthrow the “Democratic establishment.”

“I think the weaknesses of both Sanders campaigns are reflective of the weaknesses of the progressive movement more generally," said Jonathan Tasini, a progressive activist and former Sanders surrogate. "It isn’t always that the Democratic Party is corrupt and in the hands of the party elite. That is a factor, but also a factor is that you have to run effective elections."

"We’re not very good at assessing our own weaknesses," Tasini added. "People (in the movement) perceive criticism as not being loyal enough, so it blinds us from having the honest conversation that would then lead to more victories."

Rep. Rashida Tlaib, D-Mich., booed the mention of Clinton in Iowa, later apologizing publicly. Ohio state Sen. Nina Turner argued Biden had “repeatedly betrayed” black Americans, forcing Sanders to distance himself from the idea in a debate. Meanwhile, some of his endorsers with connections that stretched outside of Sanders’ wing of the party — such as former Labor Secretary Robert Reich and former NAACP President Ben Jealous — were nowhere to be seen. Rep. Alexandria Ocasio-Cortez, D-N.Y., who endorsed Sanders after his heart attack and helped turn his campaign around, stopped campaigning for him in part because the campaign publicized controversial podcaster Joe Rogan’s endorsement.

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“He was more radicalized this time than he was last time,” Longabaugh said. “One of the ways in which he was radicalized this time is not only the tenor of his campaign, but the tenor of his surrogates that he put out there.”

Insiders say the campaign didn’t do the kind of outreach it needed to do, failing to drawmembers of Congress, union leaders and black politicians into his movement in significant numbers. One campaign ally said it left would-be staffers or endorsers at a loss of how to get involved in the campaign they supported.

“I know dozens and dozens of people who would call me, who would call others up, who would say ‘I don’t know how to get through, what is going on? Why aren’t they responding to me?’" the ally said in an interview with NBC News. "This started happening through the first couple months of the race — and we figured, oh, they have a lot going on — but then it became such a pattern."

Several allies said they pushed Sanders to do more outreach, but he had a dim view of the importance of traditional endorsements and little patience for stroking the egos of politicians. So Sanders pushed on with just 10 endorsements from members of Congress or governors, just two more than he garnered during his last bid.

Vulnerable and moderate members of the House — those known as “frontliners” — recalled no outreach from the Sanders campaign, aside from one meeting with a centrist caucus in 2019. The senator won endorsements of some union leaders, but was met with criticism from others. Many sat the primary out.

Sanders allies also say a critical failure of the campaign was also his basic refusal to embrace the Democratic Party itself. In 2018, he ran for re-election to the Senate as an independent once more, instead of branding himself a Democrat.

Even by quitting the Democratic race early — a show of unity on paper as the coronavirus pandemic continues to upend traditional voting contests and campaigning — Sanders vowed to keep collecting delegates in hopes of using them to exert power over party priorities and rules at the Democratic National Convention.

“The struggle continues,” he told his supporters.