Shortly after the November election, the Trump campaign circulated on its Facebook and Twitter accounts, as well as its website, the names of seven dead Americans in the battleground states of Georgia and Pennsylvania. The dead people were used to cast votes in last month’s election, the campaign claimed, pointing to the incidents as evidence of widespread voter fraud that enabled President-elect Joseph R. Biden Jr.’s victory.

Local officials have debunked several of the dead-voter claims, and there remains no evidence of widespread voter fraud. But now, Pennsylvania officials say one of the names held up by the Trump campaign was used to cast a vote in the election.

Here’s the catch: Authorities say the fraudulent vote was cast for Mr. Trump.

This week, Jack Stollsteimer, the district attorney of Delaware County, accused Bruce Bartman of Marple Township, Pa., of illegally voting in place of his deceased mother in the general election. In addition to his mother, Mr. Bartman registered his mother-in-law, Elizabeth Weihman, who died in 2019, as a voter, according to the district attorney’s office, but is not accused of voting for her. He also cast a ballot under his own name.

“This is the only known case of a ‘dead person’ voting in our county, conspiracy theories notwithstanding,” Mr. Stollsteimer said in a statement. “The prompt prosecution of this case shows that law enforcement will continue to uphold our election laws whenever presented with actual evidence of fraud and that we will continue to investigate every allegation that comes our way.”

Samuel Stretton, a lawyer for Mr. Bartman, said: “He’s admitted everything. He’s cooperated.” Mr. Stretton added that he was negotiating a guilty plea, and that Mr. Bartman had no criminal record.

“He’s a good man,” Mr. Stretton said. “He did something very stupid under some misguided theory that this was his form of protest.”

In an interview with The New York Times in November after the Trump campaign first made its claims, Mr. Bartman said he did not recall seeing a mail-in ballot for his mother. “Oh, no, no, I haven’t gotten anything,” he said. “Occasionally I would get some junk mail for her. But not in several years.”

He added that he did not hear of the Trump campaign’s allegation because he did not use social media much and only infrequently logged on to Facebook to see pictures of his grandchildren.

Asked whether he knew why a vote for his mother would have been recorded despite her having passed away, he said the state’s governor, Tom Wolf, “doesn’t know anything or what’s going on in the city of Philadelphia, or the surrounding counties in the middle part of the state.”

“Some of the stuff that has gone on in Philadelphia is just atrocious,” Mr. Bartman added.

Mr. Stretton, his lawyer, said, “He was wrong in saying that, he admits he was wrong, and since he was approached by the detectives, he has cooperated and told the truth.”

The claim that a vote was fraudulently cast using Elizabeth Bartman’s name and that it was emblematic of systemic voter fraud helping Mr. Biden spread widely online. On Facebook, articles with the claim from the conservative websites ZeroHedge and The Epoch Times were shared 1,800 times and reached up to 61 million followers, according to data from CrowdTangle, a Facebook-owned social media analytics tool.