Justin Amash, the only Republican in Congress to support the impeachment of Donald Trump, said on Thursday that he was leaving the party and becoming an independent to fight the “partisan death spiral” of modern politics.

Amash, a congressman from Michigan, attributed his departure from the Republican party to broader concerns about two-party politics, and not strictly frustrations with Republicans.

“The two-party system has evolved into an existential threat to American principles and institutions,” Amash wrote in a Washington Post opinion piece.

He argued that the two-party system champions party loyalty above serving Americans. “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,” Amash said.

Announcing his departure from the Republican party on the Independence Day holiday, Amash issued a stern warning about what such segmentation could mean for the future of US politics. “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.”

Within two hours of his announcement, Trump criticized Amash, the only member of the Republican Congress to say Trump’s attempts to obstruct justice as outlined in special counsel Robert Mueller’s report on Russian interference in the 2016 election amounted to “impeachable conduct”. Amash did not name Trump in his article.

“Great news for the Republican party as one of the dumbest & most disloyal men in Congress is ‘quitting’ the party,” Trump tweeted. “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!”

Before accusing Trump of impeachable acts in May, Amash had an established reputation for breaking from Republicans on a variety issues. His politics lean Libertarian and he has acknowledged that he could run for president as a member of that party. Amash has not said whether he will seek a fifth term in Congress.

Amash underlined his long-held commitment to the Republican party in his piece, explaining that both his parents were Republicans. He said that his father, who immigrated from Palestine, and his mother, who immigrated from Syria, raised him to be grateful for the opportunities afforded to him as an American.

“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,” Amash said. “In recent years, though, I’ve become disenchanted with party politics and frightened by what I see from it.”

George Conway, another prominent, now-former, Republican, and the husband of Trump advisor Kellyanne Conway, supported Amash’s announcement on Twitter. Conway wrote “I couldn’t agree more,” in response to Amash’s declaration and shared a photo of his own form asking to be registered with no political party, dated March 2018.

Bill Kristol, also a prominent Republican critic of Trump, tweeted: “Amash is loyal to country over party and to the constitution rather than Trump. This higher loyalty unnerves Trump. It reminds him that many don’t respect him, and that many who pretend to respect him don’t.”

Despite Trump’s declared support for the Republican party, he has switched political parties several times and threatened to run in the 2016 election as an independent.