Gianforte, a technology entrepreneur, defeated Democrat Rob Quist to continue the GOP's two-decade stronghold on the congressional seat. Democrats had hoped Quist, a musician and first-time candidate, could have capitalised on a wave of activism following President Donald Trump's election.

Instead, the win reaffirmed Montana's voters support for Trump's young presidency in a conservative-leaning state that voted overwhelmingly for him in November.

Gianforte was a strong favourite throughout the campaign and that continued even after authorities charged him with misdemeanour assault on Wednesday. Witnesses said he grabbed Ben Jacobs, a reporter for the Guardian newspaper, and slammed him to the ground after being asked about the Republican health care bill.

Gianforte dropped out of sight after he was cited by police and ignored calls on Thursday by national Republicans for him to apologise to the reporter.

He emerged only at his victory celebration Thursday night, where he said he accepted responsibility for the incident. “Last night I made a mistake and I took an action I can't take back and I am not proud of what happened,” Gianforte told the crowd. “I should not have responded the way I did and for that I am sorry.”

The last-minute controversy unnerved Republicans, who also faced close calls this year in the traditionally Republican congressional districts in Kansas and Georgia. A runoff election is scheduled for next month in Georgia between Democrat Jon Ossoff and Republican Karen Handel after Ossoff fell just short of winning outright.

Quist told supporters that he called Gianforte to congratulate him on his win and to urge him to represent all Montanans. “I know that Montanans will hold Mr Gianforte accountable,” Quist said Thursday night.

Gianforte showed lukewarm support for Trump during his unsuccessful run for governor in Montana last fall but did an about-face and turned into an ebullient Trump supporter after he started campaigning for the congressional seat vacated by Republican Ryan Zinke, when he was tapped by Trump to serve as Interior Department secretary.

Gianforte urged Montana voters to send him to help Trump “drain the swamp,” brought in Vice President Mike Pence and first son Donald Trump Jr to campaign for him and was supported by millions of dollars of ads and mailers paid for by Republican groups.

But the theme of the election shifted Wednesday night when Jacobs walked into Gianforte's office as he was preparing for an interview with Fox News.

Jacobs began asking the candidate about the health care bill passed by the House when the crew and Jacobs say Gianforte slammed him to the floor, yelling “Get out of here!”

Gianforte's campaign issued a statement Wednesday blaming the incident on Jacobs. But on Thursday night, Gianforte apologised both to Jacobs and to the Fox News crew for having to witness the attack. “I should not have treated that reporter that way and for that I'm sorry, Mr Jacobs.”

It had been unclear if Gianforte's assault charge would impact the race. About a third of eligible voters in Montana had already cast their ballots in early voting, and others said it didn't influence their vote.

Shaun Scott, a computer science professor at Carroll College in Helena, said the assault charge was barely a factor in his decision.

“If you have somebody sticking a phone in your face, a mic in your face, over and over, and you don't know how to deal with the situation, you haven't really done that, you haven't dealt with that, I can see where it can... make you a little angry,” Scott said Thursday.

Quist, a popular 69-year-old singer and cowboy poet who was the front man for the Montana's Mission Mountain Wood Band, was helped by money that poured in from across the US as Democrats seek to capture congressional seats that would have been considered safely Republican a year ago.

But Gianforte also benefited from millions of dollars spent on ads and mailers by GOP groups like the Conservative Leadership Fund.

Gianforte campaigned as a gun-loving Montanan endorsed by the National Rifle Association to build his credibility among hunting enthusiasts and to motivate gun rights activists to vote. He echoed the Republican Party mantras of cutting taxes, beefing up the military and securing the country's borders.

Montana is a conservative-leaning state that became even more so after voters last November overwhelmingly supported Trump, voted in Republican majorities in the state Legislature and elected GOP candidates to four of five statewide elected positions, leaving Gov. Steve Bullock as the only Democratic statewide elected official.

A Democrat has not held the Montana US House seat since Pat Williams departed in 1997 after he decided not to seek re-election.

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Quist ran a nontraditional populist campaign that saw appearances by Sen. Bernie Sanders of Vermont. He stuck to issues that have broad appeal in Montana, such as maintaining and improving access to public land. He collected nearly $3.2 million from individual donors across the US.

But Quist had to overcome reports of financial problems that included unpaid taxes, a loan default and legal squabbles with a former band member over royalties and a contractor over payments. He tried to turn those negatives into positives by saying his story illustrated problems many Montanans face because of high health care costs.

Libertarian Mark Wicks was the third candidate in the race.