Democratic Primaries

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.

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.

I 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.

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.

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.)

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.”