Republican congresswoman Liz Cheney on Thursday endorsed Democratic congresswoman Elissa Slotkin of Michigan and plans to campaign for her.

It is the first time that Cheney, a critic of Donald Trump who lost her Republican primary, has crossed party lines to formally support a Democrat.

Cheney, of Wyoming, announced her support for the two-term House member from Holly, Michigan, in a statement by the Slotkin campaign that notes she plans to headline a campaign event with Slotkin in the Lansing-area district next Tuesday.

Slotkin is competing against Republican state senator Tom Barrett in Michigan’s seventh congressional district. Their race is considered a toss-up by both sides and one of the Republicans’ chief targets in their campaign to win the House majority on 8 November.

Cheney and Slotkin serve on the House armed services committee, but their shared background in the federal government goes back further. Cheney worked in the state department before launching her political career, as did Slotkin, who worked in the CIA and the defense department as well.

Both have been vocal critics of House Republicans who have sought to downplay the siege of the US Capitol on 6 January 2021. Cheney is vice-chair of the House January 6 committee, which recently issued a subpoena for Trump to testify.

“I have come to know Elissa as a good and honorable public servant,” Cheney said in a statement included in the Slotkin campaign’s announcement.

“While Elissa and I have our policy disagreements, at a time when our nation is facing threats at home and abroad, we need serious, responsible, substantive members like Elissa in Congress.”

As of two weeks ago, Michigan’s seventh district was the most expensive House race in the country, according to AdImpact, a non-partisan political media tracking company. The two campaigns and outside groups had combined to spend $27m.

Biden received more votes than Trump in the district in 2020 by less than 1 percentage point.