Mr. Shakir, who did not know that the strategists were planning to quit, had not even yet met with them. Mr. Sanders’s new team said little about the surprise exit of the consultants’ firm, known as Devine, Mulvey and Longabaugh. “The campaign appreciates all the good work DML has done and wishes them well,” said Mr. Shakir in a statement.

Privately, Mr. Sanders’s supporters were irritated and suggested the consultants walked away because they were not going to have the same influence as they did four years ago.

Mr. Longabaugh dismissed that claim, noting that advertising firms are by their nature central to modern campaigns and that Mr. Sanders would inevitably have come to rely on their long experience in Democratic politics.

The senator clashed at times with his consulting team in 2016 — particularly Mr. Devine, and particularly at the end of the race when it was clear that Mr. Sanders could not win the nomination — but Mr. Mulvey and Mr. Longabaugh had been expected to play central roles in 2020. For months, they had been making the case that Mr. Sanders should not be underestimated.

But on Tuesday, Mr. Longabaugh would not rule out the firm working for another Democratic candidate. “We haven’t talked about that so I’m not going to comment,” he said.

[Who’s in? Who’s out? Keep up with the 2020 Democratic field with our candidate tracker.]

Mr. Sanders, who struggled to win African-Americans voters, has made clear that he wanted a more diverse array of campaign advisers after largely surrounding himself with white men in 2016. The three consultants departing are all white, while Mr. Shakir will be the first Muslim in American history to run a presidential campaign.

And Mr. Sanders will hold a series of rallies this weekend that nod to his biography — a marked contrast to his 2016 campaign, when he eschewed talking about himself and relentlessly focused on his progressive policy agenda. He is appearing in Brooklyn, where he was born, and in Chicago, where he went to college and joined civil rights protests. The weekend tour will include a stop in Selma, Ala., for events marking the anniversary of Bloody Sunday there.

And while he may have a different group of advisers for this campaign, Mr. Sanders clearly retains the same grass-roots appeal: On his first day as a presidential candidate, he raised nearly $6 million, and on Monday, six days after he joined the 2020 race, campaign officials said he had secured $10 million from 359,914 donors.