Liz Cheney is a critic of former President Donald Trump.
A member of Cheney's staff told CNBC that the Wyoming Republican had endorsed a democrat. The endorsement by Cheney is his first since the Wyoming primary.
According to the Democrat's campaign, Cheney and Slotkin are going to campaign together in Michigan.
The Republican Party has been denounced by Cheney since a mob of Trump's supporters attacked the U.S. Capitol.
Cheney was censured and stripped of her leadership role because she criticized Trump. She is a Republican member of the House select committee, which is investigating the insurrection.
Slotkin, a two-term congresswoman, is fighting to keep her seat in Michigan's 7th Congressional District, where she faces a major threat from Republican State Sen. The race is rated a toss-up.
A source close to Cheney told CNBC thatBarrett is an election denier who doesn't acknowledge Trump's loss to Biden in the 2020 vote. State senators signed a letter asking Congress to investigate voter fraud claims.
Many of Trump's supportersstormed the Capitol because of his false claims about a "rigged" election. Large numbers of Republicans still question Biden's electoral victory, and a majority of GOP nominees cast doubt on the results, according to polls.
Cheney endorsed Slotkin in a statement shared by the Democrats. I have come to know Elissa as a good and honorable public servant who works hard for the people she represents and is in this for the right reasons.
At a time when our nation is facing threats at home and abroad, we need substantive members like Elissa in Congress. All voters in the 7th district should vote for her.
At a time when the very heart of our democracy is being challenged, Cheney has been a critical voice.
Slotkin said that he was grateful for her support in the race and that he was proud of the work they had done together.