Last year, Ms. Sanders helped found the Sanders Institute, a progressive think tank with a mission to “revitalize democracy.” It is, organizers insist, separate from the senator’s political campaigns.

But on Thursday night, many key figures in his orbit — including Jeff Weaver, his 2016 campaign manager; Nina Turner, now the president of the Sanders-aligned group Our Revolution; and Phil Fiermonte, a longtime aide — were in attendance, chummily holding court by the stage during the opening cocktail reception.

The actor Danny Glover was there, too, as was Cornel West, the scholar and activist. Even Mayor Bill de Blasio of New York, no doubt thrilled to be considered a progressive leader, was expected to make an appearance.

“It’s a gathering of the tribe,” Gunnar Lovelace, an entrepreneur from Los Angeles and a Sanders supporter, said as he eyed the attendees. Many had loaded their plates with Vermont cheeses and charcuterie.

Even with the hard-to-ignore undercurrent of 2020, however, the focus appeared to be more on policy than politicking. Mr. Sanders called again for the United States to end its support for Saudi Arabia in the war in Yemen, a foreign policy issue that has increasingly become a pillar of his message. He hit other high points of his stump speech, repeating his favorite lines about economic inequality and a $15 minimum wage.

“People do believe in our ideas,” he said, trumpeting his role in moving the Democratic Party to the left. “So what do we do?”

“We have got to make sure that the Democratic Party is not just a party of the east coast and the west coast,” he said, not quite taking the reins but not dropping them either. “It is a party of every state in this country.”

At the end of the night, he stepped off the stage and mingled with the audience. He did not seem like someone who wanted to leave.