Former Representative Fred Upton, the Michigan Republican who served three decades in the House and retired in 2022 after voting to impeach former President Donald J. Trump, endorsed Vice President Kamala Harris on Thursday, becoming the latest G.O.P. figure to cross party lines and vouch for her.

Mr. Upton said in a statement that the former president was “unfit to serve as commander in chief again” and blamed him for the mob attack on the Capitol on Jan. 6, 2021, “when he directly jeopardized the peaceful transition from one administration to the next.”

Mr. Upton, 71, said he had already cast his ballot for Ms. Harris — the first time in his life he had voted for a Democrat for president — and that he was confident she would work to “bring people together.”

He argued that it was long past time for Republicans to abandon Mr. Trump.

“Time and time again, respected senior national Republicans have urged our former president to focus on governing rather than personal attacks, mistruths and continued false 2020 election claims,” he said. “Instead of heeding that advice, we see unhinged behavior not acceptable in most forums almost daily.”

Ms. Harris is hoping that support from Mr. Upton, a moderate Republican in a critical battleground state, could help sway just the kind of Michigan voter she needs to peel away from Mr. Trump to win. She is working to appeal to G.O.P. voters who may have supported Nikki Haley in the primary and may still be reluctant to back Mr. Trump, but who are also on the fence about voting for a Democrat.

Ed Duggan, the Harris campaign’s Michigan state director, called Mr. Upton’s endorsement a “courageous and honorable decision to put country over party.”

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