Citing the warnings in George Washington’s farewell address, Amash said 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.”

He maintained that most Americans are not “rigidly partisan” and that the country can do better than the two-party system.

“No matter your circumstance, I’m asking you to join me in rejecting the partisan loyalties and rhetoric that divide and dehumanize us,” Amash wrote, declaring his party affiliation as independent on the nation’s Independence Day.

Amash, who was elected in the tea party wave of 2010, embraced libertarian principles and repeatedly broke with his party on major issues and even the mundane before he challenged the president.

A few hours after Amash’s declaration, Trump derided him in a tweet: “Great news for the Republican Party as one of the dumbest & most disloyal men in Congress is ‘quitting’ the Party. 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!”

House Republican leaders had no immediate response to Amash quitting the party.

Amash does not mention Trump by name in the op-ed, but his party switch comes weeks after he posted tweets arguing that former special counsel Robert S. Mueller III documented “impeachable conduct” in his report on Trump’s campaign and administration and Russian interference in the 2016 election. In subsequent public statements, Amash challenged colleagues from both parties to act.

About 80 House Democrats and several 2020 presidential candidates have called for launching impeachment proceedings against Trump, a step that House Speaker Nancy Pelosi (D-Calif.) has resisted. Amash is the only Republican calling for such action.

His stand has drawn the wrath of Trump, widespread criticism from other Republicans, and a challenger in next year’s election. Amash, 39, who is serving his fourth term, has not announced whether he will run again. He has acknowledged that he is instead considering running for president as a member of the Libertarian Party.

Michigan state Rep. Jim Lower, a Republican, announced his bid for Amash’s seat in the U.S. House of Representatives, describing himself in a news release as “a Pro-Trump, Pro-Life, Pro-Jobs, Pro-2nd Amendment, Pro-Family Values Republican.”

Last month, Donald Trump Jr., the president’s eldest son, indicated that he planned to campaign for Lower.

“See you soon Justin,” Trump Jr. said in a tweet. “I hear Michigan is beautiful during primary season.” Attached to his tweet were poll results showing Amash trailing Lower in Michigan’s 3rd Congressional District.

Past supporters of Amash also turned on him in the aftermath of his impeachment comments.

The leader of the Club for Growth, a conservative group that has staunchly backed Amash since his first congressional run in 2010, and whose spending helped him overcome a 2014 primary challenge, criticized his comments.

The congressman has been steadfast in defending his position. Facing voters at a town hall in Grand Rapids, Mich., several weeks ago, Amash said Congress “has a duty to keep the president in check.”

“I’d do it whether it was a Republican president or a Democratic president,” he said. “It doesn’t matter. You elected me to represent all of you.”

In Amash’s Michigan district on Thursday, his decision to quit the party divided his constituents, with some saying they felt betrayed after electing the Republican last November and others praising the congressman’s defiance.

David Luther, 38, a small-business owner from Kentwood, said Amash “doesn’t get along with people that disagree with him. I think Justin Amash is more interested in getting his name in the newspaper than he is in serving his constituents.”

But Chuck Richardson, 35, a Grand Rapids resident who works in IT management, said Amash was “brave for standing up for what he believes in and not going with the flow with all the other Republicans. There are a bunch of people who didn’t like Trump to begin with, and then he became president, and everyone is kissing his feet.”

Richardson said he would vote for Amash again.

House Minority Leader Kevin McCarthy (R-Calif.) has dismissed Amash as an outlier among Republicans, citing a number of instances in which the congressman voted with Democrats.

“Justin Amash can determine his own future. But I think, on a philosophical basis, he’s probably in a different place than the majority of all of us,” McCarthy said last month.

Amash also left the conservative House Freedom Caucus, a group he helped found four years ago.

It was unclear whether he would caucus with Democrats. The only other independents in Congress, Sens. Bernie Sanders (Vt.) and Angus King (Maine), caucus with Democrats in the Senate.