Other overtures were made in private. Mr. O’Rourke has held calls with the Rev. Al Sharpton, the civil rights leader, and made repeated contact with senior members of the N.A.A.C.P. In recent months, Mr. Merkley and Texas Democrat Julián Castro have reached out to Rashad Robinson, the president of Color of Change, the progressive civil rights advocacy group that has positioned itself as a go-to sounding board for prospective candidates.

Ms. Warren has stood out in her aggressive wooing of black leaders, according to several people who have spoken with members of her team. During a trip to New York City, she arranged a sit-down meeting with Mr. Robinson to discuss issues popular with social justice activists, including the elimination of cash bail and sweeping criminal justice reform.

“The person that is able to beat Trump won’t be running against Trump per se, they’ll be running for a vision of America that’s bigger than him. And that vision of America will have to speak to black voters, explicitly,” Mr. Mckesson said in a statement. Several prospective candidates reached out to Mr. Mckesson in order to appear on his podcast, including Mr. O’Rourke and lesser known presidential candidates such as Representative John Delaney of Maryland.

Mr. Robinson, the head of Color of Change, said candidates seeking to run in 2020 are not asking for endorsements or statements of support at this time. He said his meeting with Ms. Warren and others was more about discussing policy issues, rather than campaign strategy. The meetings come as several possible presidential campaigns, including that of Mr. Sanders, have made hiring more diverse campaign staffers a top priority.

“I think some of the candidates don’t get that they’re going to have to actually come with a bold agenda,” Mr. Robinson said. “If candidates think that this is just about who can beat Trump, they’re going to have a rude awakening. People can hate Trump and not love the opposition. It actually has to be about putting out something that allows people to feel like they are connected and their community’s issues are being addressed.”

The behind-the-scenes activity signals just how different this upcoming primary stands to be for Democrats. Not only are multiple black candidates expected to run, including Senators Cory Booker of New Jersey and Kamala Harris of California, but several white candidates are confident they can successfully compete for minority votes.