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By Justin Mitchell

MISSOULA, Mont. (Reuters) - A Montana Republican running for the U.S. Congress has been charged with assaulting a reporter hours before polls were to open on Thursday for a special election that could test U.S. President Donald Trump's political clout.

The incident on Wednesday threatened to roil a tightening race in the Republican-leaning state, where a Democratic political novice aimed to pull off a victory in a contest seen as a bellwether for next year's U.S. congressional elections.

It was not clear what effect the assault charge against Republican technology executive Greg Gianforte would have on the Montana race, where 37 percent of the 699,207 registered voters had already submitted absentee ballots, according to state election officials.

Gianforte was charged with misdemeanor assault after a political correspondent for the U.S. edition of the Guardian newspaper said the candidate had "body-slammed" him during a campaign event in Bozeman.

Ben Jacobs was trying to ask Gianforte about healthcare, according to an audio tape captured by the British newspaper's correspondent. He was taken to the hospital and later released, media reports said.

Fox News Channel reporter Alicia Acuna, who was preparing to interview Gianforte at the time, said the candidate "grabbed Jacobs by the neck with both hands and slammed him to the ground."

Acuna, her field producer and photographer then "watched in disbelief as Gianforte began punching the reporter, she wrote on the Fox News website.

"I'm sick and tired of you guys," Gianforte can be heard saying in the audio tape. "The last guy who came here did the same thing. Get the hell out of here."

Gianforte was favored in a state where Republicans have held its lone House seat for two decades and where fellow Republican Trump won by more than 20 percentage points in the 2016 presidential election.

He faces Democrat Rob Quist, a banjo-playing folk singer and first-time candidate, to fill the U.S. House of Representatives seat vacated when Trump named Ryan Zinke as secretary of the interior. Quist declined to comment on the incident.

Gianforte's campaign did not deny Jacobs' allegation but countered in its own statement that the reporter provoked an altercation by barging into the candidate's office, shoving a recording device in his face and "asking badgering questions."

Gianforte has until June 7 to appear in a county court. He faces a $500 fine and six months in jail if convicted, the Gallatin County Sheriff’s Office said.

Three local newspapers have since withdrawn their endorsements for Gianforte.

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Quist has focused his campaign on sharply criticizing the Republican effort to repeal and replace the Affordable Care Act, former Democratic president Barack Obama's signature healthcare law known as Obamacare.

According to the audio tape, Jacobs' encounter with Gianforte turned violent when he tried to ask the candidate if he supported a Republican healthcare overhaul bill after the Congressional Budget Office found the measure would cost 23 million Americans their medical insurance coverage by 2026.

Gianforte campaign spokesman Shane Scanlon said in a statement that Jacobs declined to lower his recorder when asked. "Greg then attempted to grab the phone that was pushed in his face. Jacobs grabbed Greg's wrist and spun away from Greg, pushing them both to the ground.

"It's unfortunate that this aggressive behavior from a liberal journalist created this scene at our campaign volunteer BBQ," the statement said.

Acuna disputed that Jacobs was the aggressor, writing: "At no point did any of us who witnessed this assault see Jacobs show any form of physical aggression toward Gianforte."

A Democratic upset in the race would set off alarms for Republicans already worried about Trump's unpopularity and the healthcare issue. In the November 2018 elections, Republicans must defend their 24-seat House majority.

Gianforte's defeat would also give Democrats momentum heading into two special House elections for Republican-held seats next month, in Georgia and South Carolina. Republicans had to sweat out a closer-than-expected special House election win in conservative Kansas last month.

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