“Climate change affects everything in my life, from where I’m going to live to whether I wanted to have kids,” said Mr. Gardner, who added that he did not know many details about Mrs. Clinton’s policy agenda.

Before Mr. Sanders took the stage, several celebrity supporters addressed one of his biggest challenges in New York: He has been a far more popular vote-getter than Mrs. Clinton with independents, yet only registered Democrats are allowed to cast ballots in the primary. They urged the crowd to do everything they could to persuade more Democrats to support Mr. Sanders and help turn out the vote on Tuesday.

Mrs. Clinton, who will debate the senator on Thursday, has held a comfortable lead over him in recent opinion polls. But Sanders supporters at the rally said they thought the intensity of support for the senator could lead to an upset victory on Tuesday, similar to his win in Michigan last month.

“Change will not happen by choosing a candidate who is entirely entrenched in the dysfunction of the past,” said the actor Tim Robbins, who recalled protesting in the park against the Vietnam War as a young man.

Mr. Sanders told the crowd that “it’s going to be a tough primary for us” because independents could not vote and young people, among others, could not register at polling places on Tuesday. But he said he was still hopeful.