A Republican congressman from Montana has announced that he is quitting politics after he says his failed recent bid for the US senate led to “a death threat” against him as well as “false and defamatory rumors” about him and his family.

Matt Rosendale, a member of the far-right House freedom caucus, signed up in February to participate in Montana’s Republican primary to challenge the Democratic incumbent, Jon Tester, in November. But Rosendale soon withdrew after Donald Trump and the National Republican Senatorial Committee each endorsed his opponent, Tim Sheehy, an aerospace millionaire and retired navy seal.

He initially planned to seek re-election to the seat he has held since 2021 but encountered political headwinds. Notably, his office threatened to sue the former Democratic senator Heidi Heitkamp after she appeared on the 26 February episode of the Talking Feds with Harry Litman podcast and accused Rosendale of bailing from the Senate primary because he had impregnated a staff member.

In addition to a lawsuit threat aimed at Heitkamp, Rosendale’s office responded with a statement calling the congresswoman’s allegation “100% false and defamatory”.

Rosendale, 63, had another prominent detractor in Marjorie Taylor Greene, a fellow rightwing extremist member of Congress who endorsed Sheehy in the Republican primary to challenge Tester. Greene called Rosendale a “grifter” and insinuated that he wasn’t truly loyal to Trump.

On Friday, Rosendale said he had intended to pursue re-election to his congressional post “at the urging of many, including several of the current candidates”. But since then, his statement said, “I have been forced to have law enforcement visit my children because of a death threat against me and false and defamatory rumors against me and my family”.

Rosendale’s statement didn’t elaborate but added: “This has taken a serious toll on me and my family.

“The current attacks have made it impossible for me to focus on my work to serve you. So, in the best interest of my family and the community, I am withdrawing from the House race and will not be seeking office.”

Rosendale’s term expires in early 2025. Those who have signed up to run for his seat in November include Denny Rehberg, who from 2001 until 2013 served as what was then Montana’s lone congressional representative.

The brief Senate run that preceded Rosendale’s political retirement was his second attempt at Tester’s seat. He ran against Tester in 2018 but lost by a little more than three percentage points.

Rosendale had previously served as Montana’s state auditor and in its legislature. He was among the group of conservative insurgents who made former the Republican US House speaker Kevin McCarthy endure 15 rounds of votes before he was given the chamber’s gavel in early 2023.

McCarthy retired last year after being ousted as House speaker in October. His replacement as speaker was the Louisiana Republican Mike Johnson, who at one point planned to endorse Rosendale’s more recent Senate run before reportedly changing his mind when faced with backlash from within their party.