And nowhere were these competing demands on display more than in the contretemps with Mr. Biden.

In the interview, Mr. Booker targeted Mr. Biden with a new and direct ferocity. He argued that black voters should demand answers from the former vice president on issues including his 1970s opposition to busing and the tough-on-crime legislation he wrote in the 1990s, as well as his use of the word “boy” last week in recalling what the segregationists he served with called him.

“Explain your stance against busing, explain what you were thinking with the 1994 crime bill, explain what you meant when your language dredged up something like that,” Mr. Booker said.

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Yet even as he voiced this indictment of Mr. Biden, he softened it on the front and back end.

“I love the guy,” he said after recounting what he said was a very positive telephone conversation with Mr. Biden last week.

And after laying out his bill of particulars, Mr. Booker was quick to add of the former vice president: “Doesn’t mean we don’t love and respect him.”

His supporters have tried to put the best face on his early difficulties, but they recognize that he must use the debate Wednesday and then the one in July to stand out from the field.

“It’s not as great as I would have hoped at this point, and I’m sure that Cory feels the same way,” said Tracy Higgins, a Booker donor based in New Jersey who is planning to host a fund-raiser for him this fall. “But it’s tough with so many candidates, and I think that for Cory, he’s not yet really been able to make his more specific case about ‘why him.’ The debate can certainly present that opportunity.”