Rep. Justin Amash, who left the Republican Party, announces his candidacy for U.S. Senate in Michigan.

Amash joins a crowded field of Republicans vying for the battleground Senate seat that is being vacated by the retirement of Democratic Sen. Debbie Stabenow. Other Republican candidates include Mike Rogers, another former Michigan congressman who is backed by the Senate Republican campaign arm.

Mr. Amash represented Michigan’s 3rd Congressional District from 2011 to 2021. He started out as a Republican but emerged as a vocal critic of Trump, leaving the party in 2019 to declare himself an independent and supporting Trump’s first impeachment later that year.

Amash joined the Libertarian Party in 2020, becoming the first Libertarian to serve in Congress. After running for president on the Libertarian Party, he decided not to seek reelection to his House seat.

Amash announced the creation of a review committee for the Senate seat in January, criticizing the current candidates as “uninspired, unserious and unprepared” to tackle the nation’s challenges.

In addition to Mr. Rogers, the primary race also includes former Michigan congressman Peter Meyer and businessman Sandy Pensler. Former Detroit Police Chief James Craig was also a candidate, but withdrew this month.

Meyer, also a critic of Trump, voted for his second impeachment and lost his 2022 re-election campaign to a Trump-backed primary candidate.

The Michigan Democratic Party responded to Amasi’s announcement by highlighting the “brutal infighting” within the state’s Republican Party. LaVora Burns, the state party’s president, predicted in a statement that the Republican Party’s “acrimonious showdown will leave a deeply damaged candidate unable to contact his family in Michigan.”