Republican congressional candidate Greg Gianforte
Republican congressional candidate Greg Gianforte speaks to supporters during a campaign event on May 23, 2017. Getty

A Republican candidate for the Montana congressional election has allegedly "body-slammed" a journalist a day before voters go to the polls.

Greg Gianforte is up against the Democrats' Rob Quist to fill the seat left empty when former Congressman Ryan Zinke joined President Donald Trump's cabinet as Secretary of the Interior. The two candidates were head-to-head in the race.

At a campaign event for Gianforte on Wednesday (24 May), he allegedly assaulted Guardian political journalist Ben Jacobs.

Jacobs, who has released an audio recording of the altercation, asked Gianforte about healthcare, but the candidate seemed reluctant to give an answer.

When Jacobs pressed the issue, a loud crashing noise could be heard before Gianforte shouted: "I'm sick and tired of you guys. The last time you came here, you did the same thing. Get the hell out of here."

A shocked Jacobs could be heard saying: "Jesus Christ ... You just body-slammed me and broke my glasses."

Alexis Levinson, a Buzzfeed reporter, witnessed part of the altercation.

"This happened behind a half-closed door, so I didn't see it all, but here's what it looked like from the outside," she wrote on Twitter.

"Ben walked into a room where a local TV crew was set up with an interview with Gianforte.

"All of a sudden, I heard a giant crash and saw Ben's feet fly in the air as he hit the floor."

Jacobs then called police and officers from the Gallatin County Sheriff's Office were pictured speaking to Gianforte outside the event. Gianforte then drove away, over an hour before his own event was due to finish.

In a statement from the Gianforte campaign, a spokesman denied the allegations and blamed the "aggressive behaviour from a liberal journalist".

The Gallatin County Sheriff's Office confirmed it was investigating allegations of assault against Gianforte and said it would provide further details at a press conference scheduled for 8pm local time (4am GMT).

A month before the alleged incident, Jacobs had published a story which revealed Gianforte's financial ties to a number of Russian companies.