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"Today, I am declaring my independence and leaving the Republican Party," Amash wrote. "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."
Having been brought up by Republican immigrant parents, Amash wrote, he supported Republican candidates throughout his early adult life and believed that the party "stood for limited government, economic freedom and individual liberty," but has become "disenchanted" with party politics and is "frightened" by what he has seen from it in recent years.
"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," Amash wrote. "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."
Trump later responded in a tweet, calling Amash a "total loser."
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!— Donald J. Trump (@realDonaldTrump) July 4, 2019
In a May Twitter thread, Amash laid out his own "principal conclusions" from the 448-page Mueller report, including that "Attorney General [William] Barr has deliberately misrepresented Mueller's report," and that "President Trump has engaged in impeachable conduct."
Here are my principal conclusions:— Justin Amash (@justinamash) May 18, 2019
1. Attorney General Barr has deliberately misrepresented Mueller’s report.
2. President Trump has engaged in impeachable conduct.
3. Partisanship has eroded our system of checks and balances.
4. Few members of Congress have read the report.
The special counsel did not establish that the Trump campaign or anyone associated with it conspired or coordinated with Russia. He also provided no conclusion on the matter of possible obstruction of justice, choosing instead to leave that decision for Congress.
Trump responded in a tweet at that time, saying he was "never a fan" of Amash.
Never a fan of @justinamash, a total lightweight who opposes me and some of our great Republican ideas and policies just for the sake of getting his name out there through controversy. If he actually read the biased Mueller Report, “composed” by 18 Angry Dems who hated Trump,....— Donald J. Trump (@realDonaldTrump) May 19, 2019
Amash renouncing his ties to the Republican Party could have implications on Capitol Hill. He could potentially lose his membership in the Republican conference as well as his seat on the House Oversight Committee, which has investigative powers over government activities, including open lawsuits and investigations against Trump and his administration.
Running for re-election as independent could also pave the way for another Republican to run with the party’s nomination for his House seat in 2020.
A pro-Trump Michigan state Rep. Jim Lower had already launched a primary challenge against Amash, and the president's son, Donald Trump Jr. hinted a few weeks ago that he would campaign against the former Michigan Republican.
Other congressional Republicans have come after Amash and defended the president, while he has been winning praise from Democrats, who see Amash as the first break in the GOP opposition to possible impeachment, allowing them to claim the effort is bipartisan.
Amash left the conservative House Freedom Caucus last month.
ABC News' Benjamin Siegel contributed to this report.