BOZEMAN, Mont. — A Republican congressional candidate in Montana charged with shoving a reporter to the ground on the eve of a special election kept a low profile Thursday even as supporters prepared a hotel ballroom for a possible victory party.

Greg Gianforte was charged with misdemeanor assault after witnesses said he grabbed Ben Jacobs, a reporter for The Guardian who is originally from Baltimore, by the neck on Wednesday and threw him to the ground at Gianforte’s campaign headquarters in Bozeman.

Gianforte, a millionaire tech entrepreneur who has aligned himself with President Donald Trump, said Jacobs, a 2002 graduate of Park School who grew up in Mount Washington, was being aggressive and grabbed him by the wrist at his campaign office.

The altercation occurred hours before voters went to the polls to decide whether Gianforte or Democrat Rob Quist will fill the U.S. House seat vacated by Ryan Zinke, a Republican who is now Trump’s interior secretary.

Many see the closely watched election as a referendum on the policies and practices of Trump.

Democratic Gov. Steve Bullock, who defeated Gianforte in November’s election, called Wednesday’s events “another wake up call to all Montanans and Americans that we must restore civility in politics and governing.”

Republicans have held the state’s lone congressional seat for two decades.

It wasn’t clear how the last-minute melee would affect the race. More than a third of the state’s registered voters had cast absentee ballots before polls opened Thursday.

Three of Montana’s biggest newspapers pulled their endorsements of Gianforte — without endorsing his opponent — while leaders of both major parties called on him to apologize.

House Speaker Paul Ryan, a Republican, said what occurred was “wrong and should not happen.” House Minority Leader Nancy Pelosi, a Democrat, called Gianforte “a wannabe Trump.”

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

As Jacobs pressed the candidate 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 Gianforte then began to punch Jacobs.

In an audio recording posted by The Guardian, the reporter asks the congressional candidate about the health care bill.

“We’ll talk to you about that later,” Gianforte says on the recording, referring Jacobs to a spokesman.

When Jacobs says there won’t be time, Gianforte says “Just--” and there is a crashing sound. Gianforte yells, “The last guy who came here did the same thing,” and a shaken-sounded Jacobs tells the candidate he just body-slammed him.

“Get the hell out of here,” Gianforte says.

Jacobs’ mother, Hillary Jacobs, a Mount Washington resident, said she heard about the Wednesday night incident on Twitter — “the way everyone finds everything out these days.

She said the audio of the alleged assault was captured on Ben Jacobs’ cellphone, so for a while he was not communicating by phone. But at one point, she said, he was able to text to say he was at the hospital but not to worry.

“So I kept that in mind, however, hearing the audio over and over again was more than a little disconcerting,” she said. “It’s something that you don’t necessarily expect to happen to a reporter. But obviously, based on all the coverage that’s occurred since the incident, everyone is surprised that this kind of incident occurred.”

She said she has spoken with her son only briefly because he is working, covering the election in addition to becoming part of the story.

Baltimore Sun reporters Lorraine Mirabella and Sean Welsh contributed to this article.