“If they thought that Joe Biden was the best choice, maybe they shouldn’t have run,” Rasheen Aldridge, a Missouri state representative who was a warm-up speaker for Mr. Sanders in St. Louis, said in an interview. “Some of these candidates — Kamala has been out for a long time, Booker has been out for a while. It’s all very kind of fishy how things are working out.”

The 2016 campaign established Mr. Sanders as the Democratic Party’s foremost practitioner of grievance politics. Like Donald Trump in that year’s Republican primary, Mr. Sanders tapped into populist anger to upend a major political party.

Then, Mr. Sanders denounced Democratic National Committee debate rules he said favored Hillary Clinton’s campaign and a superdelegate system he argued empowered party insiders over grass-roots voters. His vociferous, public complaints led the D.N.C. to enact substantive changes to the rules for caucus states and to strip first-ballot voting power from superdelegates at the party’s convention.

Now the Sanders campaign says the policies he’s pitching as a democratic socialist — “Medicare for all,” free public college tuition and other proposals — represent an existential threat to powerful operators that have prompted an unusually coordinated effort to block his rise.

“We face a unique uphill battle that other candidates don’t because of the agenda that he stands for,” a campaign spokesman, Mike Casca, said. “Because of the agenda that he’s putting forward, a lot of super wealthy forces are aligned against him.”

Yet others close to the Sanders campaign say he didn’t expect to find himself trailing Mr. Biden at this point in the race and is trying to save face with his most ardent supporters by blaming others. It’s also not bad for fund-raising — perhaps uniquely among the 2020 candidates, Mr. Sanders does just as well with online contributions after he loses states than after he wins.

The ability to keep raising tons of money can allow Mr. Sanders to prolong his candidacy even if voters keep delivering a verdict more favorable to Mr. Biden.