“The neoliberalist who all of a sudden now is coming back to life, and the catalyst was my own black people. Oh, I’m so disappointed,” he said. “What has happened to our black leadership? Some have just sold out.”

The decision to let the panelists provide the message was an unusual one, particularly for a candidate who has cast his campaign as a multiracial coalition that can mobilize a movement of working-class Americans.

“He didn’t want to speak on behalf of people of color when there were people of color on the panel,” said Mike Casca, a campaign spokesman. “Bernie does not have those experiences. He’s a white Jewish man.”

The event itself was an acknowledgment that Mr. Sanders is still struggling to improve his standing among black voters four years after he lost his first run for president in part because of his inability to gain their support.

On Super Tuesday last week, Mr. Biden outperformed Mr. Sanders by 40 points or more among black voters in Texas, North Carolina and Virginia. In several states, Mr. Sanders came in third among black voters, behind not only Mr. Biden but also Michael R. Bloomberg.

In Flint, Mr. Sanders drew only a smattering of nonwhite voters to his Saturday night event, advertised as a “racial and economic justice town hall” with the candidate.