Federal prosecutors charged Vincent C. Gray’s campaign chauffeur with campaign finance fraud Thursday, bringing the long-running investigation closer to the outgoing D.C. mayor.

Mark H. Long, 47, is expected to plead guilty to conspiring with D.C. businessman Jeffrey E. Thompson and others to conceal under-the-table campaign payments and participating in a scheme to pay off another mayoral candidate to exit the race.

Billy Martin, Long’s attorney, declined to comment on the charges, which were filed in D.C. Superior Court.

In March, Thompson pleaded guilty in federal court to funding a “shadow” effort to help the Democratic mayor win the 2010 mayoral election by pumping more than $660,000 into the campaign. Long becomes one of the closest campaign aides to the mayor to be linked with allegations of accepting illegal funds from Thompson. He worked as Gray’s personal driver and served as his head of advance operations during the 2010 campaign.

Gray, who is traveling in China, has denied wrongdoing since the investigation began more than three years ago. Robert S. Bennett, his attorney, said Thursday he had not yet reviewed the charges against Long and declined to comment.

Long is scheduled to appear Friday before Judge Anita Josey-Herring. Two people involved in the case said that Long is expected to plead guilty. The felony conspiracy count carries a maximum of five years in prison.

Allegations contained in Thursday’s court filing largely mirror previous reports involving Gray, Thompson and others, but they also include new details.

The Washington Post reported last year about discussions involving Gray, consultant Jeanne Clarke Harris and Leo Alexander, another candidate in the 2010 Democratic primary, about arranging for Alexander to drop out of the race before the election. The charges against Long include prosecutors’ first mention of those conversations, including a “secret meeting” in Maryland between Gray and Alexander that Long and Harris — a close Thompson associate and godmother to Long — allegedly arranged.

Prosecutors allege that Gray pursued a scheme to offer Alexander “a thing of value” in exchange for dropping out of the race and endorsing Gray. According to court papers, Gray told Long that Alexander wanted a position inside Gray’s administration.

Prosecutors also said a $20,000 check issued from the Gray campaign to Harris was “intended to be used” to encourage Alexander to drop out of the race. But Alexander did not leave the race, and prosecutors say Harris then returned the $20,000 to Gray’s campaign as a “refund.”

Alexander reiterated Thursday that he was not offered, nor did he accept, any money to drop out of the 2010 race.

“We weren’t doing anything illegal,” said Alexander, who said he was interviewed multiple times by FBI agents. “We just had conversations. There’s nothing wrong about having conversations.”

The Post also reported in 2012 that Long and the SUV in which he chauffeured Gray were funded not through the Gray campaign, but through Harris, who pleaded guilty to wrongdoing in 2012. The charging papers filed Thursday say Harris’s consulting firm — using funds Thompson provided — paid Long’s salary and for the SUV.

Before joining Gray’s campaign, in 2008, Long ran for an at-large seat on the D.C. Council. That campaign, according to the charging papers and people familiar with the federal investigation, was one of several Thompson secretly funded in violation of campaign finance laws.

Gray, referred to in the court papers as “Mayoral Candidate A,” allegedly knew of Long’s association with Thompson and Harris. Long, they say, drove Gray to two meetings with Thompson, delivered donations from the businessman to Gray, and facilitated telephone calls between the two.

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