The Montana Republican Greg Gianforte plans to plead guilty to a misdemeanor assault charge for “body-slamming” the Guardian reporter Ben Jacobs, according to the Gallatin county attorney Marty Lambert.

Lambert told the Associated Press that Gianforte would plead guilty when he appeared in court for arraignment and sentencing on Monday. Misdemeanor assault carries a maximum penalty of a $500 fine or six-month jail sentence.

The Guardian was not immediately able to reach Lambert or Gianforte for comment.

A tech millionaire, Gianforte was charged following an altercation on the eve of a special election to fill Montana’s sole seat in the US House of Representatives.

Jacobs had approached Gianforte to ask a question about the Republican healthcare bill when the then candidate threw him to the ground and began punching him. Though the Gianforte campaign initially blamed Jacobs for the altercation, an audio recording and the first-hand account of one of three Fox News reporters who witnessed the assault contradicted the campaign’s statement.

“Gianforte grabbed Jacobs by the neck with both hands and slammed him into the ground behind him,” wrote the Fox News reporter Alicia Alcuna. “Faith, Keith and I watched in disbelief as Gianforte then began punching the reporter.”

Gianforte was charged later that night and emerged the winner in the contested election the next day.

On Wednesday, Gianforte and Jacobs reached a settlement to preclude any civil claims stemming from the assault. Gianforte issued a direct apology to Jacobs, writing in a letter that the “physical response to your legitimate question was unprofessional, unacceptable, and unlawful”. Gianforte also agreed to pay $50,000 to the freedom of the press group the Committee to Protect Journalists.

Jacobs signed a release foregoing a civil lawsuit and agreed not to object if Gianforte pleaded no contest to the criminal charges.

A no-contest plea allows a defendant to be sentenced without entering a guilty plea, which could be used against him in a civil case.

The Democratic Congressional Campaign Committee immediately seized on the political implications of the guilty plea.

“Now that Greg Gianforte will admit to being a violent criminal, the case could not be more clear: Greg Gianforte is unfit to represent the people of Montana, and should decline to take the oath of office,” the DCCC spokesperson Drew Godinich said in a statement.

The two parties poured $17m into the race for the seat, which was vacated by the secretary of the interior, Ryan Zinke. Gianforte faced off against the Democrat and political novice Rob Quist, who was well-known around the state for his folk singing career.

While the criminal case is expected to be resolved on Monday, Gianforte could still face consequences in Washington. On 2 June, a coalition of freedom of the press organizations lodged a complaint with congressional ethics officials seeking discipline for Gianforte.

The groups also wrote to Donald Trump to express concern about the president’s rhetoric against the press.

“A clear and unequivocal rejection of attacks on the press would be welcomed by political officials on both sides of the aisle, and is a necessary corrective to the corrosive atmosphere created by your earlier rhetoric,” the groups wrote.