President Trump critic Justin Amash quits Republican Party

Congressman Justin Amash of Michigan's Third District

(CNN) - Rep. Justin Amash, the only congressional Republican who publicly argues that President Donald Trump has engaged in impeachable conduct, announced Thursday he is quitting the GOP.

"Today, I am declaring my independence and leaving the Republican Party," Amash wrote in a Washington Post op-ed Thursday morning. "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 didn't mention the President by name, but his decision to abandon the party comes after months of escalating criticism not just of the President but of his own colleagues for their failure to hold Trump to account, specifically for the actions detailed in special counsel Robert Mueller's report on the 2016 election and its aftermath.

In May, the five-term Michigan representative became the first sitting Republican lawmaker to join Democratic calls for an impeachment inquiry. He drew scorn from GOP leaders and Trump — Amash has "been a loser for a long time," the President responded — when he wrote in a series of tweets that, after reading Mueller's report on Russian interference in the 2016 election, he believed Attorney General William Barr intentionally misrepresented Mueller's findings and Trump engaged in obstruction of justice.

Amash, a staunch libertarian, litigated the failures of the Republican Party, Congress and the American political system as a whole in his Post op-ed.

"The Republican Party, I believed, stood for limited government, economic freedom and individual liberty — principles that had made the American Dream possible for my family," he wrote. "In recent years, though, 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."

He also pointed to George Washington's farewell address, in which the first US President warned Americans of the dangers of partisanship.

"True to Washington's fears, Americans have allowed government officials, under assertions of expediency and party unity, to ignore the most basic tenets of our constitutional order: separation of powers, federalism and the rule of law," Amash said. "The result has been the consolidation of political power and the near disintegration of representative democracy."