“The margin by which we lost the African-American vote has got to be — at the very least — cut in half or there simply is no path to victory,” the team wrote in the memo, which was reviewed by The New York Times. Mr. Sanders had won 14 percent of the black vote there compared with 86 percent for Hillary Clinton, according to exit polls.

Over seven pages, the team outlined a strategy for winning black voters that included using social media influencers and having Mr. Sanders give a major speech on discrimination in a city like St. Louis or Cincinnati…

Dissatisfaction among black staff members on the 2016 campaign was reported at the time, including by Fusion. But fresh interviews show that many former black employees still feel frustrated that they were not taken seriously or provided with the resources they needed to succeed — even though some continue to admire Mr. Sanders.

“It’s almost like they didn’t need us for anything,” said John Solomon, who was hired as an organizer but soon became disillusioned when campaign aides assigned him to drive people around Iowa in a truck. “It was kind of like, you have black staff just to say you have black staff.”