A staffer for Herschel Walker’s Republican Senate campaign filed a lawsuit against the prominent conservative activist Matt Schlapp on Tuesday, accusing Schlapp of groping him during a car ride in Georgia before last year’s midterm election.

Schlapp denies the allegation. His lawyer said they were considering a countersuit.

The battery and defamation lawsuit was filed in Alexandria circuit court in Virginia, where Schlapp lives, and seeks more than $9m in damages.

It accuses Schlapp of “aggressively fondling” the staffer’s “genital area in a sustained fashion” while the staffer drove Schlapp back to his hotel from a bar in October, on the day of a Walker campaign event.

The allegations were first reported by the Daily Beast.

The staffer filed the lawsuit anonymously as “John Doe”, citing his status as an alleged sexual assault victim and fearing backlash from supporters of Schlapp, a longtime adviser to Donald Trump and chair of the American Conservative Union, which organizes the Conservative Political Action Conference.

The lawsuit accuses Schlapp and his wife, Mercedes, who was Trump’s White House director of strategic communications, of defamation and conspiracy, citing Matt Schlapp’s repeated denials and alleged attempts by both to discredit the staffer.

In a statement, the lawyer Charlie Spies accused the staffer of trying to harm the Schlapp family.

“The complaint is false and the Schlapp family is suffering unbearable pain and stress due to the false allegation from an anonymous individual,” Spies said, adding that the legal team was “assessing counter-lawsuit options”.

According to the lawsuit, Matt Schlapp was in Georgia in the waning days of the general election season to campaign for Walker, the former University of Georgia football star who lost a runoff election to the Democratic senator Raphael Warnock. The lawsuit alleges that after the staffer drove Schlapp back to Atlanta following a campaign event, Schlapp invited the staffer to join him for drinks.

The two ended up at a local bar, where Schlapp, according to the suit, “sat unusually close” to the accuser, “such that his leg repeatedly contacted, and was in almost constant contact with Mr Doe’s leg”.

The staffer said he offered to drive Schlapp back to his hotel but during the car ride Schlapp allegedly placed his hand on the staffer’s leg without consent, leaving him “frozen with shock, mortification and fear from what was happening, particularly given Mr Schlapp’s power and status in conservative social circles”.

When they reached the hotel, the suit alleges, Schlapp invited the staffer up to his room, but the staffer declined. He later informed senior campaign aides about what had happened and recorded a video recounting the events.

“As a direct and proximate result of Mr Schlapp’s battery upon Mr Doe, Mr Doe suffered damages, including without limitation shock, mortification, horror, humiliation, and distress,” the suit reads.

A former campaign official said the staffer had done everything right, including notifying his superiors, and said the campaign acted swiftly to make sure he knew he was believed.

The staffer had been scheduled to drive Schlapp to an event the next morning, but was advised to inform Schlapp alternative transportation had been arranged, according to the filing. Schlapp did not attend the event and would not have been welcome, said the official, who spoke on condition of anonymity.

Timothy Hyland, a lawyer representing the staffer, sent a statement from his law firm calling Schlapp a “sexual predator”.

“Our client takes no joy in filing this lawsuit,” it said. “However, Mr Schlapp has had ample time to accept responsibility and apologize for his despicable actions. But instead of doing the right thing, Mr Schlapp, Ms Schlapp, and their friends and associates embarked on a ridiculous, spurious and defamatory attempt to smear the reputation of Mr Schlapp’s victim.”