Clint Keaveny took his place in the stands behind President Biden at a rally on Friday in Madison, Wis., and held up a sign that read, “Pass the torch, Joe.” Before the rally was over, Mr. Keaveny had lost his role with a Democratic congressional campaign.

The event was the first in Mr. Biden’s weekend blitz of campaign events in must-win states to rebuff critics after his halting debate performance on June 27. As the president took the stage, in full view of cameras, Mr. Keaveny unveiled his poster, made of taped-together printer paper and stuffed in his waistband. It had been written by Mr. Keaveny’s mother, he said.

The moment quickly went viral. His boss noticed.

Before Mr. Keaveny had even left the event, Kristin Lyerly, the Wisconsin congressional candidate for whom he was a communications volunteer, had seen the images circulating online. She called him and asked that he part ways with the campaign, to which he agreed.

“He held up a sign that was inconsistent with the values and the ideals of our campaign,” Dr. Lyerly said in an interview. “I was just so profoundly disappointed that I called him right then and there.”

Mr. Keaveny, 27, said that Mr. Biden has been a great president, but he does not believe that he can defeat former President Donald J. Trump in November.

“It pains me to feel like a black sheep,” Mr. Keaveny said. “But I believe in following my conscience.”

The episode with Mr. Keaveny comes as questions over loyalty to President Biden in the Democratic Party are breaking out into the open and some are calling for a new nominee. Mr. Biden has repeatedly pledged to stay the course.

Dr. Lyerly is running for a vacant seat that represents a swath of northeast Wisconsin, including Green Bay, a competitive district that leans Republican. Representative Mike Gallagher, a Republican, resigned from the seat in April.