“I invite you to watch me talk about systemic racism not only when I’m talking to mostly black audiences, but when I’m talking to mostly white audiences,” Mayor Pete Buttigieg of South Bend, Ind., told the crowd. Mr. Buttigieg is seeking to shed his reputation as a candidate for white elites, and pointed to his recently released “Douglass Plan” to address racial inequality as evidence of his campaign’s growth.

Mr. Booker said his identity as a black man living in Newark gave him a unique perspective on the nation’s racial disparities that no president has had before. Ms. Harris noted that she sponsored a marijuana decriminalization bill this week, aimed at reversing the racial disparities in sentencing for drug-related crimes.

Former Representative Beto O’Rourke of Texas, who is seeking to jolt his flagging campaign, drew chuckles from the crowd for sidestepping pointed yes or no questions in favor of longer, impassioned answers. “The biggest misconception about me is that our chances have narrowed and diminished in this race,” he said.

And Mr. Biden promoted his criminal justice plan, which is designed to reduce the mass incarceration that disproportionately harms black people and rolls back some of the tough-on-crime measures he helped put into law in the 1980s and ’90s. Mr. Biden will also speak at the National Urban League conference this week in Indianapolis, in a sign of how crucial black voters are to his campaign’s chances.

“I love you,” Mr. Biden said on Wednesday. “Look me over. I need your help!”

In a preview of the coming scrutiny that Mr. Biden will face, several people at the N.A.A.C.P. convention seized on his criminal justice plan to question the former vice president’s record.

Jesse Jackson, the civil rights leader who opposed several of Mr. Biden’s crime bills in the 1980s and ’90s and ran against him for the 1988 Democratic presidential nomination, said Mr. Biden “has some responsibility for the criminal justice system as it exists now.”

Mr. Booker took a more direct shot at Mr. Biden’s plan: “For a guy who was an architect of mass incarceration, this is an inadequate solution.”