Montana special election: Voters go to the polls while Republican candidate Greg Gianforte is charged with assault

The election is being watched as a referendum on Donald Trump's performance

Greg Gianforte has been charged with assaulting a reporter (Getty)

Voters are going to the polls in a closely watched special election, in which one of the candidates has been charged with assault after allegedly attacking a journalist.

The contest for Montana’s only congressional seat was triggered after the sitting House Representative, Ryan Zinke, quit his job to become Donald Trump’s secretary of the Interior Department. The race pitches Republican Greg Gianforte against Democrat Rob Quist.

Yet on the eve of polling, the race received an unexpected jolt of drama when Mr Gianforte was charge with assault after allegedly body-slamming Ben Jacobs, a reporter from the Guardian, who had been trying to question him about the Republican healthcare proposal and the estimated number of people who would lose coverage under it.

“He took me to the ground,” Mr Jacobs told his newspaper, as he was taken to hospital in the back of an ambulance. “I think he whaled on me once or twice. He got on me and I think he hit me. This is the strangest thing that has ever happened to me in reporting on politics.”

Mr Jacobs’ account of what happened has been supported by an audio recording of the incident and the words of a Fox News journalist, Alicia Acuna, who was present at the time and who later wrote: “Gianforte grabbed Jacobs by the neck with both hands and slammed him into the ground behind him.”

Mr Gianforte, a tech millionaire, has disputed this version of events.

His campaign issued a statement blaming the incident on Mr Jacobs, saying he “aggressively shoved a recorder in Greg’s face and began asking badgering questions”. The newspaper issued its own statement, backing Mr Jacobs and his behaviour.

The contest has received national attention, coming hard on the heels of another special election in Georgia, which was also seen as a potential referendum on Mr Trump’s performance in the White House.

In that contest, there was speculation Democrat Jon Ossoff might be able to win an April 18 primary by securing more than 50 per cent of the vote. As it was, he fell a few points short and will now take part in a June 20 runoff against Republican Karen Handel.

Mr Gianforte, who has tried to align himself with Mr Trump, lost a race against Montana’s Democratic governor in November, while the New York tycoon won the the state by 20 points. The Associated Press said that in the congressional race, Mr Gianforte has been boosted by visits from Vice President Mike Pence and Donald Trump Jr.

House Minority Leader Nancy Pelosi said the congressman had failed to show any dignity and called him “a wannabe Trump”.

“I viewed that as a mum and a grandmother,” Ms Pelosi said. '“We try to have some level of dignity as to how we treat people and who we are. To see this person, who wants to be the one representative into the House of Representatives from Montana be sort of a wannabe Trump.”

House Speaker Paul Ryan, the most senior Republican on Capitol Hill, called on Mr Gianforte to apologise. and said that what took place was never acceptable, regardless of the details.

A poll by Gravis released earlier this week, before the alleged assault took place, gave Mr Gianforte a 14 point lead.

Gallatin County Sheriff Brian Gootkin announced shortly before midnight on Wednesday that a charge of assault had been filed against Mr Gianforte.

If convicted, Mr Gianforte would face a maximum $500 fine or 6 months in jail. He has until June 7 to appear in court to answer the charge.