HuffPost spoke to more than three dozen Sanders aides, allies and critics about why the progressive leader stumbled. Many of them requested anonymity to speak freely.

The answers they suggested are myriad. He failed to erect a campaign nimble enough to overcome the built-in challenges he was bound to face from a skeptical press corps and a hostile party establishment. He hung his electoral success on the relatively risky bet that he could both expand the electorate and do so in a way that would benefit him disproportionately. His staff feuded unnecessarily with Elizabeth Warren, and he failed to make inroads with older Black voters ― a repeat of 2016 dynamics.

Perhaps most significantly, Sanders failed to expand his core bloc of support into a coalition capable of winning a majority, and he did not adequately prepare for the prospect that moderates would consolidate behind Biden.

“There was a strategy to get to 30% and not to 50%,” one Sanders ally said.....

Of course, Sanders’ professed theory of the case did not require him to muse about how he might reach voters outside his demographic and ideological comfort zones. That’s because he insisted that his fortunes rested on bringing in new voters. He planned to expand the electorate by increasing turnout among an allegedly untapped bloc of young and working-class voters with a progressive worldview.

“The mythical voter that stayed home, but if you just give them a progressive candidate, they’ll show up ― it’s a lot like the tooth fairy, you hear about it a lot, but you never see it,” Mikus said. .....

In the end though, Sanders’ posture as a left-wing candidate likely did more to hurt his chances among South Carolina’s older Black voters than any single decision his campaign made.

Older Black Democrats, especially in the South, typically prioritize a Democrat’s electability above all else, since they believe protecting the freedoms they have won depends on keeping Republicans out of power, according to Darby. Biden’s perceived electability vis-a-vis the more radical Sanders was thus a critical factor.

Although Sanders racked up wins in the first three states to vote, he never managed to convince Black voters in South Carolina that he was the strongest contender against Trump in November. Clyburn’s endorsement of Biden three days before the primary was particularly powerful in this respect, according to polling.

“If the house is on fire, you put out the fire first, then you figure out what to do with the house,” Darby said. “I think Joe was the best person to put out the fire.”