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“A lot of us are just shaking our heads, saying it seems to be a disqualifying sentiment,” said Aimee Allison, founder of She the People, a political advocacy group focused on women of color. “At this point in the primary, he does not have an easy and powerful political response to a question like that — it’s got a lot of us just saying, ‘O.K., well, there are other candidates who are seriously contending for our votes.’”

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Both Ms. Daughtry and Ms. Allison noted that a recent survey of black women showed “other/prefer not to answer” leading the field, followed by Mr. Biden. They took it as a sign of how fluid the race is.

“If I were the vice president’s team, I’d double down on locking in the vote that they think they have, because right now I’d call it soft,” Ms. Daughtry said.

At the debate, Ms. Davis referred to a remark Mr. Biden had made in 1975, when he said, “I’ll be damned if I feel responsible to pay for what happened 300 years ago.” On Sunday, at the church, he struck a sharply different note.

“There can be no realization of the American dream without grappling with the original sin of slavery,” he said.

Eric Holder, the former attorney general under Barack Obama, told David Axelrod, Mr. Obama’s former chief strategist, that he doesn’t think there is any basis “for people to believe that a President Biden would be less committed to civil rights enforcement” than Mr. Obama was, according to a tweet from Mr. Axelrod, who hosts a podcast and a television show on CNN.

Throughout his 20-minute address, Mr. Biden was at times booming as he linked slavery, the bombing at the 16th Street church and the rise of white supremacy today to the nation’s centuries-long struggle with racism and oppression. At other times he spoke slowly and emotionally, as he discussed his personal experiences with tragedy. Mr. Biden, who is practiced at delivering eulogies, read prepared text from a black binder, producing a far more fluent speech than he typically delivers on the stump, when he often walks away from Teleprompters.