Michigan Representative Justin Amash, the only Republican in Congress who said President Trump engaged in impeachable conduct, announced on the 4th of July that he's leaving the GOP. Amash warned that modern politics "is trapped in a partisan death spiral" but said there is "an escape."

"Today, I am declaring my independence and leaving the Republican Party," Amash wrote in an op-ed for the Washington Post. "No matter your circumstance, I'm asking you to join me in rejecting the partisan loyalties and rhetoric that divide and dehumanize us. I'm asking you to believe that we can do better than this two-party system — and to work toward it. If we continue to take America for granted, we will lose it."

Amash said in the op-ed that both of his parents are immigrants, and he grew up supporting the GOP because he believed the party "stood for limited government, economic freedom and individual liberty."

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"In recent years, though, I've become disenchanted with party politics and frightened by what I see from it," Amash wrote. "The two-party system has evolved into an existential threat to American principles and institutions."

Mr. Trump wasted no time in hitting back, without basis calling Amash one of the "dumbest" members of the GOP.

"Great news for the Republican Party as one of the dumbest & most disloyal men in Congress is 'quitting' the Party," the president tweeted Thursday morning. "No Collusion, No Obstruction! Knew he couldn't get the nomination to run again in the Great State of Michigan. Already being challenged for his seat. A total loser!"

Amash's op-ed made no direct mention of Mr. Trump or anyone else in the Republican Party, and it did not solely blame the GOP for partisanship. Amash said leaders of both parties are distracting and dividing the public "by exploiting wedge issues and waging pointless messaging wars."

Amash in May became the first — and so far, the only — congressional Republican to say Mr. Trump's conduct met the "threshold for impeachment." The five-term congressman posted his conclusions on Twitter after reading the Mueller report, which detailed actions Mr. Trump took that may have constituted obstruction of justice. Amash also said Attorney General William Barr — who cleared the president of potential obstruction charges — "deliberately misrepresented" the Mueller report's findings.

"In fact, Mueller's report identifies multiple examples of conduct satisfying all the elements of obstruction of justice, and undoubtedly any person who is not the president of the United States would be indicted based on such evidence," Amash wrote in one tweet.

Amash's criticism of the president earned him a standing ovation from voters in his district at a town hall — along with criticism and alienation from his fellow Republicans. Mr. Trump responded by calling Amash a "loser" and "total lightweight."

Amash has not ruled out running for president in 2020 as a libertarian, though he didn't address any such plans in the op-ed.