Michael Kearney Jr., a community organizer from Charleston, S.C., admits that he isn’t crazy about Kamala Harris’s background as a prosecutor. But after President Biden dropped out of the presidential race and endorsed her as his replacement, Mr. Kearney set his misgivings aside — and opened his wallet.

“I’m all the way in,” declared Mr. Kearney, 41, who said he sent $20 to Ms. Harris’s campaign this week.

Nathan Gaines, a 43-year-old barber and Army veteran from Milwaukee, said he had been leaning toward voting for Donald J. Trump — he calls the former president “Big T” — despite having supported Democrats in every election since Barack Obama became president.

But the prospect of Harris at the top of the ticket was making him rethink that switch. Mr. Gaines now says he is open to at least considering her. “If Kamala steps up, that would change history,” he said. “I’d give her a shot.”

Black voters propelled Mr. Biden’s candidacyin 2020. And many remained loyal to him even after his dismal debate performance last month. But, like other Americans, their doubts about his ability to lead the party’s ticket were growing before he bowed out of the race on Sunday.