Supporters like Ms. Manbeck could hold the key to Democratic unity as the party tries to regroup after its stunning loss to Mr. Trump deprived it of all control in Washington. If Mr. Sanders declines a run in 2020, they will have to decide if they can get behind a new progressive champion in the Democratic Party; if they will accept the eventual nominee, whoever it is; or if they will take their passions — and votes — elsewhere.

Mr. Sanders’s wife, Jane, said in an interview that he had not ruled out running again and had remained active, traveling and advocating policies that help working-class people. “Ageism is the last ‘ism’ that seems to be acceptable to people, and I never felt that it was whether somebody was too young or too old,” Ms. Sanders said. “You win some. You lose some. And you keep on going and maybe you can win the next one.”

He would, after all, be only a year older than former Vice President Joseph R. Biden Jr., who has also made noises about running. And Mr. Trump, the oldest American to assume the presidency, will himself turn 74 in 2020.

But away from the boisterous People’s Summit in Chicago this weekend, some Sanders fans conceded their worries and suggested that the senator should focus on backing younger, fresh-faced candidates to push the Democratic Party leftward.

Max Weiss, a 19-year-old sophomore at the University of Illinois at Urbana-Champaign and the communications director for the Democratic Party chapter on campus, said he hoped Mr. Sanders would not run because it would be unhealthy for the party, which badly needs new faces.