“They basically orchestrated a coup of the party,” said Mr. Weaver, 30. “And here I was thinking the Democrats were the good guys.”

“I loved Bernie, but after he dropped out, there was this vacuum,” said Ms. Malanij, 30. “Now I’m trying to figure out what to do with all this energy.”

But surveys suggest the vast majority of Sanders supporters, as many as nine in 10, intend to vote for Hillary Clinton. And many of those who continue to rebuff her “were not truly Democrats to begin with,” said Patrick Murray, director of the Polling Institute at Monmouth University. “These were third-party voters who got pulled to the Democrats by Bernie,” Mr. Murray said. “Now they’re drifting back Green.”

The Green Party’s modern heyday — or low point, depending on the perspective — came in 2000, when its candidate, Ralph Nader, received 2.7 percent of the vote, the most the party has achieved in a national race. He also drew the ire of many Democrats after George W. Bush defeated Al Gore by a mere 537 votes in the decisive state of Florida. There, Mr. Nader tallied more than 97,000 votes, most of which, presumably, would have otherwise gone to Mr. Gore.

Donald J. Trump, the Republican nominee, has even championed the Green Party “because I figure anyone voting for Stein is going to be for Hillary,” he said recently in Toledo, Ohio.

For the Greens, the spoiler concept is sensitive territory. “We don’t want to be a threat; we want to be a force for good,” said Julie George-Carlson, 58, a longtime Green activist who is running for secretary of state of Missouri. To this day, Ms. George-Carlson said, neighbors accost her in the grocery store, blaming her for Mr. Bush’s victory in 2000. Still, she has no regrets.

“I vote for my favorite candidate,” she said. “How dare anyone tell me to do otherwise.”

But the fear is there.