Republican Greg Gianforte called his opponent in Montana's special election a "Nancy Pelosi in a cowboy hat" on Monday.
The businessman, who ran for governor unsuccessfully in 2016, said on Fox News that money from outside the state was pouring into this race from those that are out of touch with Montana's politics, including his opponent musician Rob Quist.
Mr. Gianforte is running to fill Interior Secretary Ryan Zinke's House seat in Montana. He will face Mr. Quist, who has been endorsed by Sen. Bernie Sanders of Vermont and has seen a surge in donations the past few weeks.
President Trump's son Donald Trump Jr. was in the state campaigning with Mr. Gianforte over the weekend. Mr. Gianforte said the pair had "over 2,000 people" at their rallies.
"Donald Trump won here by over 20 points. The Trump train is still chugging in Montana," said Mr. Gianforte.
Montana favors Republican candidates, but the strong turnout from Democrats in the special elections in Kansas and Georgia have given the party hope in Big Sky Country.
The special election is set to take place on May 25.