League of Conservation Voters officials said they are spending $244,000 on ads through April, the first significant outside group spending in Gianforte's nascent re-election campaign.
They acknowledged the ad takes a shot at Gianforte's assault conviction but said Gianforte's actions since he's been in office are no joke.
"Those words are used intentionally because of the gravity of the situation," said Clayton Elliott, head of the league's Montana chapter. "These are serious efforts to remove protections from the places we hold dear."
Eight candidates, including six Democrats, are vying for the chance to unseat Gianforte this fall, after the Montana businessman won a special election last May to fill the remainder of Ryan Zinke's congressional term. On the eve of that election, Gianforte knocked Guardian reporter Ben Jacobs to the ground as Jacobs attempted to ask him a question.
Gianforte pleaded guilty to the misdemeanor charge.
Gianforte spokesman Travis Hall did not immediately respond to a request for comment Wednesday.
Montana State University political science professor David Parker said he knows of no other group spending money in Montana's House race yet. Outside groups would have to see the race as winnable before they're likely to spend money to influence it, and nobody has emerged yet as a clear front-runner to challenge Gianforte.
"I wouldn't be spending that money here unless I had solid evidence this race was flippable," Parker said.
The League of Conservation Voters officials insists the ad campaign is not about Gianforte's re-election, but about bills pending in Congress.
The group is critical of Gianforte for his vote supporting a measure that would limit the power of the president to designate national monuments. Gianforte voted with other House Natural Resource Committee Republicans who want to require state and local approval before the president can designate monuments larger than 15 square miles.
The group also opposes legislation sponsored by Gianforte to remove federal protections from more than 1,000 square miles (2,590 sq. kilometers) of land in Montana that have been designated as wilderness study areas for decades.
Gianforte introduced his bills earlier this month, saying he was seeking the release of land that had been set aside to study for possible wilderness areas. Those studies were completed years ago, but Congress never took action to remove the protections that limit access and development to those areas, Gianforte said in remarks on the House floor last week.
The conservationist group's ad accuses Gianforte of "attacking our way of life and assaulting our public lands and national monuments" and repeats that he's "attacking our national monuments and public lands." It ends with a directive to Gianforte to "stop voting to assault our public lands."
The ad will run on video and radio streaming services, along with news and social media websites. Additionally, users of the mapping app Waze will receive an anti-Gianforte pop-up ad when they are traveling near certain public lands in Montana - and near Gianforte's district offices.
Copyright 2018 The Associated Press. All rights reserved. This material may not be published, broadcast, rewritten or redistributed.