Jeffrey Cooper, 51, who was waiting for a bus at the Olney Transportation Center in North Philadelphia, suggested that the best nominee to close that gap would be Mr. Biden.

“Joe Biden has a connection with the African-American people,” said Mr. Cooper, a computer programmer. “If Joe Biden comes here personally — walks among the people — if he does that a few times, he’ll get the votes of the people.”

The competition for votes is early, of course: Ms. Harris and Mr. Booker have yet to start aggressively campaigning in the state or courting black voters, and Mr. Booker may benefit in Philadelphia by pointing to his work in neighboring New Jersey. But for now, Representative Dwight Evans, whose district in Philadelphia is majority African-American, said of Mr. Biden, “I think of all the candidates — nothing against any of those other candidates — he is in the best position, and has the best skill set necessary to become the next president.”

Mr. Biden is targeting a swath of voters — including older, less educated and less liberal Democrats — who are often ignored by other candidates chasing younger voters more in keeping with the leftward energy in the party.

[Check out our tracker of the 2020 Democratic candidate field.]

And to many Democrats, the worst-case scenario in a field of historic diversity would be the nomination of a septuagenarian white male moderate. If activists coalesce behind alternatives to Mr. Biden once primary voting gets underway next spring, the landscape could look much different than today.

Still, a Morning Call/Muhlenberg College poll of Pennsylvania this month showed that 45 percent of Democrats identified as moderate or conservative, and they strongly favored Mr. Biden over his closest rival, Mr. Sanders. Voters older than 50, a majority of registered Democrats, prefer Mr. Biden to Mr. Sanders by blowout margins. Over all, Mr. Biden led Mr. Sanders in the statewide poll of registered Democrats, 28 percent to 16 percent.