Michigan Rep. Justin Amash announced Thursday that he was leaving the GOP after growing "disenchanted" and "frightened" by party politics.
"Modern politics is trapped in a partisan death spiral, but there is an escape," said Amash, who in May became the first congressional Republican to conclude that President Donald Trump has engaged in "impeachable conduct."
His decision came after reading former special counsel Robert Mueller's redacted report on Russian interference in the 2016 election, Amash tweeted in a widely circulated thread.
Trump hit back at Amash on Twitter at the time, calling him a "loser."
On Thursday, Trump said Amash's decision to leave the party was "Great news."
Amash, 39, resigned from the conservative House Freedom Caucus last month.
He has clashed with Trump and the Republican Party at various points.
First elected to Congress in 2010 as part of the tea party wave focused on a limited government and ballooning deficits, he repeatedly clashed with his party on issues of government spending, health care and finally impeachment.
Amash's call for impeachment proceedings against Trump drew widespread criticism from Republicans and sparked questions about his political future.
His tweets on the issue led to Amash receiving his first official 2020 GOP primary challenger — Michigan state Rep. Jim Lower, who said in a statement that he is a “pro-Trump, pro-life, pro-jobs, pro-Second Amendment, pro-family values Republican.”
Standing by his position late last month, he told a town hall of voters that he was worried about losing the trust of the public. He cited volume II of the Mueller report that outlines nearly a dozen instances where Trump attempted to interfere with the investigation.
Amash did not say whether he would caucus with the Republicans in future. He has previously said he would not rule out a presidential campaign against Trump.
In his July Fourth op-ed, Amash wrote that Congress is no longer serving its functions as envisioned by the country's founders.
"We are fast approaching the point 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," said Amash, the son of a Palestinian immigrant.
Amash added: "Most Americans are not rigidly partisan and do not feel well represented by either of the two major parties. In fact, the parties have become more partisan in part because they are catering to fewer people, as Americans are rejecting party affiliation in record numbers."
"No matter your circumstance, I’m asking you to join me in rejecting the partisan loyalties and rhetoric that divide and dehumanize us," Amash added. "If we continue to take America for granted, we will lose it."