LANSING, Mich. (NEXSTAR) — A retiring Michigan congressman is leaving the Republican Party and becoming an independent to protest efforts to overturn Democrat Joe Biden’s victory over President Donald Trump.

Rep. Paul Mitchell, of Dryden, is nearing the end of his second and final term. He wrote a letter to Republican National Committee Chairwoman Ronna McDaniel and House Minority Leader Kevin McCarthy on Monday, the same day electors formalized Biden’s win.

“Today I am disaffiliating from the Republican Party,” Mitchell tweeted, along with a copy of the letter.

He said it’s “unacceptable for political candidates to treat our election system as though we are a third-world nation and incite distrust of something so basic as the sanctity of our vote.”

Mitchell, 64, said he voted for Trump despite some reservations about four more years of his leadership. He criticized GOP leaders for tolerating “unfounded conspiracy theories” without defending a secure electoral process. He did not join 106 House Republicans who signed onto a Texas lawsuit — rejected by the U.S. Supreme Court — that sought to invalidate the results in Michigan and three other states.

“I believe that raw political considerations, not constitutional or voting integrity concerns, motivate many in party leadership to support the ‘stop the steal’ efforts, which is extremely disappointing to me,” Mitchell wrote. “As elected members of Congress, we take an oath to ‘support and defend the Constitution of the United States,’ not to preserve and protect the political interests of any individual, be it the president or anyone else, to the detriment of our cherished nation.”

Biden noted during a speech Monday in Wilmington, Delaware, that Trump called his 2016 tally of 306 electoral votes a “landslide.”

Biden says if that constituted a clear victory then, he wanted to “respectfully suggest” that Trump now accept Biden’s victory this year.

Trump has refused to concede defeat in the presidential vote, repeating unfounded allegations of widespread fraud.