WASHINGTON – U.S. Rep. Justin Amash of west Michigan quit the Republican Party on Thursday, saying that extreme partisanship in Washington and across the U.S. has "evolved into an existential threat to American principles and institutions."

In an opinion piece written for the Washington Post, Amash — who is the only Republican in Congress to call for President Donald Trump's impeachment — said he would serve as an independent and called on other Americans to abandon party labels and reject "the partisan loyalties and rhetoric that divide and dehumanize us."

His office confirmed later in the morning that he plans to run for reelection in his Republican-leaning district, centered near Grand Rapids, as an independent.

"The founders envisioned Congress as a deliberative body in which outcomes are discovered," Amash wrote in the opinion piece. "We are fast approaching the point, however, where Congress exists as little more than a formality to legitimize outcomes dictated by the president, the speaker of the House and the Senate majority leader."

"Modern politics is trapped in a partisan death spiral, but there is an escape," he said.

At midnight, Amash, now I-Cascade Township, also posted on Twitter a photo of the Declaration of Independence with the note, "Happy Birthday, America."

Possibly because the surprise announcement was made early on a holiday morning, reaction was slow to come in, though Trump weighed in, calling Amash's departure "Great news for the Republican Party" and calling the congressman "one of the ... most disloyal men in Congress."

Amash didn't comment directly on his political future in the piece but a reporter for Michigan Radio and NPR caught up with him at a parade in Grand Rapids and he said he planned to run for reelection as an independent.

"I've been thinking about it for awhile," Amash said. His office confirmed his decision a short time later.

With several other Republicans and Democrats already in the race, Sabato's Crystal Ball, a political handicapping site at the University of Virginia's Center for Politics, declared the seat up for grabs if Amash, a five-term incumbent, ran as an independent.

In earlier remarks, Amash had also not ruled out a run for president, though neither had he indicated very strongly that he was looking to make such a run. His decision to run for reelection appeared to make that unlikely.

In May, Amash became the first — and so far only — Republican in Congress to call for Trump's impeachment, saying he reached that conclusion after reading the entirety of Special Counsel Robert Mueller's report into Russian interference in the last presidential election and possible obstruction of justice claims.

Amash, a lawyer, later returned to Twitter to lay out those claims in full, saying, in part, the Mueller report "describes a consistent effort" by Trump to obstruct or impede the investigation "because it put his interests at risk."

Trump — who has wrongly claimed that the Mueller report exonerated him — went on Twitter to respond, calling Amash a "loser" and a "lightweight."

It wasn't the first time Trump and Amash had sparred but it marked a clear line between Amash — who has always had an independent streak compared to establishment Republicans — and his party.

Several people, including Peter Meijer, heir to a well-known grocery chain founder, state Reps. Jim Lower of Greenville and Lynn Afendoulis of Grand Rapids, and former Sand Lake Village President Tom Norton announced they would run for the Republican nomination against Amash following his taking a stance against Trump.

Amash, 39, who was first elected to Congress as part of the Tea Party wave in 2010 and was a state legislator before that, didn't mention Trump by name in his opinion piece on Thursday. Instead, he focused on a level of partisanship in the country — and a centralizing of political power in Washington among partisan leaders, a complaint he has made before — that he says is damaging civil discourse and democracy.

He said that while he has supported Republican candidates and principles of "limited government, economic freedom and individual liberty" throughout his life, he has "become disenchanted with party politics and frightened by what I see from it."

Reiterating George Washington's deep concerns about the dangers of partisanship, Amash said, "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.

"The result has been the consolidation of political power and the near disintegration of representative democracy.".

Other points made by Amash in the piece:

"With little genuine debate on policy happening in Congress, party leaders distract and divide the public by exploiting wedge issues and waging pointless messaging wars. These strategies fuel mistrust and anger, leading millions of people to take to social media to express contempt for their political opponents, with the media magnifying the most extreme voices. This all combines to reinforce the us-vs.-them, party-first mind-set of government officials."

He referred to "a mind-set among the political class that loyalty to party is more important than serving the American people or protecting our governing institutions. The parties value winning for its own sake, and at whatever cost. Instead of acting as an independent branch of government and serving as a check on the executive branch, congressional leaders of both parties expect the House and Senate to act in obedience or opposition to the president and their colleagues on a partisan basis."

"In this hyperpartisan environment, congressional leaders use every tool to compel party members to stick with the team, dangling chairmanships, committee assignments, bill sponsorships, endorsements and campaign resources. As donors recognize the growing power of party leaders, they supply these officials with ever-increasing funds, which, in turn, further tightens their grip on power."

Amash said he believes that most Americans "are not rigidly partisan and do not feel well-represented by either of the two major parties" and that, in fact, record numbers of people are rejecting partisan labels.

"These same independent-minded Americans, however, tend to be less politically engaged than Red Team and Blue Team activists," he said. "Many avoid politics to focus on their own lives, while others don’t want to get into the muck with the radical partisans.

"But we owe it to future generations to stand up for our constitutional republic so that Americans may continue to live free for centuries to come. ... 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."

More on freep.com:

10 things about Justin Amash, the Michigan Republican who wants Trump impeached

Peter Meijer, heir to grocery chain fortune, to take on Justin Amash

Contact Todd Spangler:tspangler@freepress.com. Follow him on Twitter@tsspangler. Read more onMichigan politics and sign up for ourelections newsletter.