Representative Justin Amash of Michigan, the only sitting Republican member of Congress to support impeaching President Trump, announced on Thursday that he was leaving the party after facing fierce attacks from the president and fellow Republicans.
In an op-ed essay in The Washington Post that did not mention Mr. Trump by name, Mr. Amash wrote: “I’ve become disenchanted with party politics and frightened by what I see from it. The two-party system has evolved into an existential threat to American principles and institutions.”
Three hours after the essay was published, Mr. Trump responded with a personal attack against Mr. Amash, calling him “one of the dumbest and most disloyal men in Congress.”
Mr. Amash, 39, is known as a libertarian with a contrarian streak and has been one of Mr. Trump’s staunchest critics on the right. He has even considered a run against him in the 2020 election. Mr. Amash’s move on Thursday makes him the only independent member of the House, which has 235 Democrats and, now, 197 Republicans.
In May, he became the first — and so far the only — sitting Republican member of Congress to join Democrats in saying that the president had committed offenses that rose to the level of impeachment.
That assertion was based on his reading of the redacted report by the special counsel, Robert S. Mueller III, which was released in April. In a series of tweets, Mr. Amash accused Attorney General William P. Barr of deliberately misrepresenting the report’s findings in his summary. Mr. Amash argued that the report had provided multiple examples of conduct that could be labeled obstruction of justice.
The president immediately struck back, calling Mr. Amash a “loser” and reinforcing the congressman’s isolation within the Republican Party. A conservative state representative in Michigan, Jim Lower, and a National Guard veteran, Tom Norton, quickly suggested that they might mount primary challenges if Mr. Amash runs for a sixth term next year.
In his essay, published on the morning of Independence Day, Mr. Amash wrote that his father, a Palestinian refugee who moved to the United States at 16, had instilled in him the belief that America is a land of opportunity.
Mr. Amash quoted George Washington on the dangers of partisanship and strongly criticized the two-party system.
“Modern politics is trapped in a partisan death spiral, but there is an escape,” he wrote.
He called for Americans to join him “in rejecting the partisan loyalties and rhetoric that divide and dehumanize us.”
More coverage of Justin Amash’s clashes with President Trump and the G.O.P.