Advisers to the senator maintain that the African-American outreach team had plenty of resources and was a victim of its own inefficiency.

“The black people had the largest team — the Latino outreach team didn’t even have that many people,” said Symone Sanders, who took over black outreach efforts after the South Carolina primary. “And then what did they do with the resources? They squandered it.”

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.”

This tension spilled over into other areas of the campaign, particularly among local staff members in South Carolina, who worked closely with the African-American outreach team. Some black campaign workers described microaggressions — subtle interactions that, while not overtly discriminatory, still played on racial prejudice. One woman said her boss almost never spoke to her.

[Who’s in, who’s out: Keep up with the Democratic field with our candidate tracker.]

Mr. Sanders appears committed to strengthening ties with the black community since his loss to Mrs. Clinton. He has made multiple trips to the South, and last month he spent Martin Luther King’s Birthday in South Carolina, where he spoke at the Capitol and met with black leaders, elected officials and students.