WASHINGTON — Rep. Justin Amash, R-Mich., a critic of President Donald Trump who has entertained a run against him in 2020, became the first Republican congressman to say the president “engaged in impeachable conduct” based on the Mueller report.

Often the lone Trump dissenter on his side of the House aisle, Amash shared his conclusions in a lengthy Twitter thread Saturday after reviewing the full report by special counsel Robert Mueller.

Amash wrote that after reading the 448-page report, he had concluded that not only did Mueller’s team show Trump attempting to obstruct justice, but that Attorney General William Barr had “deliberately misrepresented” the findings.

He added that “few members of Congress even read Mueller’s report.”

“Contrary to Barr’s portrayal, Mueller’s report reveals that Trump engaged in specific actions and a pattern of behavior that meet the threshold for impeachment,” Amash wrote.

The White House did not respond to a request for comment.

Trump often claims the report shows “no collusion, no obstruction,” though neither is true. Mueller did not establish a conspiracy between the Trump campaign and Russia, which interfered in the 2016 election.

Mueller did not rule on the question of obstruction of justice, saying it was something Congress should determine.

Amash wrote that it was partisanship keeping Republicans from exercising Congress’ checks and balances role.

“When loyalty to a political party or to an individual trumps loyalty to the Constitution, the Rule of Law — the foundation of liberty — crumbles,” he tweeted.

Amash, a libertarian, considers himself a strict constitutionalist and in February was the lone Republican to join a Democratic bill to stop Trump from declaring a national emergency to fund his border wall.

“From the time the president was elected, I was urging them to remain independent and to be willing to push back against the president where they thought he was wrong,” Amash told CNN in March. “They’ve decided to stick with the president time and again, even where they disagree with him privately.”Elected in 2010 during the tea party wave, Amash co-founded the House Freedom Caucus, which at the time devoted itself to issues such as repealing the Affordable Care Act. Since Trump’s election, the group has morphed into a mouthpiece for the president on Capitol Hill.

When Trump mocked former congressman Mark Sanford last year after he lost his House race in South Carolina’s Republican primary, Amash called it a “dazzling display of pettiness and insecurity.” Trump has not hit back.