Montana Republican congressional candidate Greg Gianforte greets voters while campaigning for a special election in Missoula, Montana, U.S. May 24, 2017 in this still image from video. REUTERS/Justin Mitchell

A candidate in a US congressional election has been charged with misdemeanor assault for grabbing a reporter by the neck and throwing him to the ground.

Greg Gianforte, the Republican candidate in the Montana election, would face a maximum fine of 500 US dollars (£385) or six months in jail if convicted.

Gallatin County Sheriff Brian Gootkin made the announcement in a written statement shortly before midnight on Wednesday, about six hours after the attack on reporter Ben Jacobs of The Guardian.

The statement added that Mr Jacobs' injuries did not meet the legal definition of felony assault.

There was a local TV crew there when Gianforte body slammed me. Audio is posting soon at @GuardianUS — Ben Jacobs (@Bencjacobs) May 24, 2017

Mr Gianforte was in a private office preparing for an interview with Fox News when Mr Jacobs came in without permission, campaign spokesman Shane Scanlon said.

The Fox News crew watched in astonishment as, after Mr Jacobs pressed him on the GOP health care bill, "Gianforte grabbed Jacobs by the neck with both hands and slammed him into the ground behind him," Fox News reporter Alicia Acuna wrote in an article.

She added that Mr Gianforte then began to punch Mr Jacobs.

In an audio recording posted by the Guardian, the reporter asks the congressional candidate about the GOP's health care bill, which was just evaluated hours earlier by the Congressional Budget Office.

"We'll talk to you about that later," Mr Gianforte says on the recording, referring Mr Jacobs to a spokesman.

When Mr Jacobs says that there will not be time, Mr Gianforte says "just -" and there is a crashing sound.

Mr Gianforte yells "the last guy who came here did the same thing", and a shaken-sounded Mr Jacobs tells the candidate he just body-slammed him.

"Get the hell out of here," Mr Gianforte says.

The incident is a last-minute curveball in Thursday's race, which was partly seen as a referendum on Donald Trump's presidency.

The majority of voters were expected to have already cast ballots through early voting, and it was unclear how much of an effect the assault charge would have on the election results.

Mr Gianforte and Democrat Rob Quist, who declined to comment, are seeking to fill the state's seat in the US House left vacant when Ryan Zinke resigned to join Mr Trump's Cabinet as secretary of the Interior Department.

Mr Gianforte, a wealthy businessman, lost a race against Montana's Democratic governor in November while Mr Trump won the state by 20 points.

In the congressional race, Mr Gianforte has tried to tie himself to the president and been boosted by visits from Vice President Mike Pence and Donald Trump Jr.

Hours before Wednesday's assault, the Gianforte campaign sent out a last-minute fundraising appeal to its supporters, saying the outcome "will determine whether we pass Donald Trump's America First agenda or if the fake news media and the national Democrats will win, keeping Obama's reckless policies in place".

Democrats were hoping an upset would send a message to the GOP that Mr Trump's souring approval ratings could damage their political fortunes even in deep red states.

The Democratic Congressional Campaign Committee announced that it would launch as many Facebook ads as possible about the assault, targeting Montana Democrats who might not otherwise vote on Thursday.

The Committee called for Mr Gianforte to quit the race and for the Republican Party to denounce him publicly.

The Guardian's US editor Lee Glendinning said in a statement: "The Guardian is deeply appalled by how our reporter, Ben Jacobs, was treated in the course of doing his job as a journalist while reporting on the Montana special election.

"We are committed to holding power to account and we stand by Ben and our team of reporters for the questions they ask and the reporting that is produced."

Online Editors