Rep. Justin Amash of Michigan became the first Republican lawmaker to say President Trump had "engaged in impeachable conduct." In a series of tweets on Saturday, Amash wrote he had read the full report by special counsel Robert Mueller and had concluded the president's conduct met the "threshold for impeachment."

"Under our Constitution, the president 'shall be removed from Office on Impeachment for, and Conviction of, Treason, Bribery or other high Crimes and Misdemeanors,'" Amash wrote Saturday. "While 'high Crimes and Misdemeanors' is not defined, the context implies conduct that violates the public trust."

The president responded to Amash on Sunday morning, calling him a "loser" and "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."

Justin Amash, left, and President Trump. Getty

Amash wrote that impeachment does not require "probable cause" has been committed, but rather "simply requires a finding that an official has engaged in careless, abusive, corrupt or otherwise dishonorable conduct."

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After Mueller concluded his report in mid-March, Attorney General William Barr released a four-page summary, which Amash said "deliberately misrepresented" the full 448-page report. "Barr's misrepresentations are significant but often subtle, frequently taking the form of sleight-of-hand qualifications or logical fallacies, which he hopes people will not notice," he wrote.

Amash wrote that, contrary to Barr's initial assessment of Mueller's report, Mr. Trump had "engaged in specific actions and a pattern of behavior that meet the threshold for impeachment.

Congress, too, was not spared from criticism by Amash. He called out the partisanship of many lawmakers, saying they shift their perceptions "180 degrees" when discussing former President Bill Clinton and Mr. Trump.

"Few members of Congress even read Mueller's report; their minds were made up based on partisan affiliation — and it showed, with representatives and senators from both parties issuing definitive statements on the 448-page report's conclusions within just hours of its release," Amash wrote.

Amash, who has been described as the "new Ron Paul" due to his Libertarian beliefs, has broken ranks with the Republican party before — and especially against Mr. Trump, whom he did not endorse in 2016. Earlier this year, he was one of 14 Republican representatives who tried to override Mr. Trump's veto over an emergency declaration for the border wall. In March, Amash did not rule out running for president as a Libertarian in 2020.

Several of the Democrats running for president have called for Mr. Trump's impeachment.

Read the full tweet thread below:

Here are my principal conclusions:

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. — Justin Amash (@justinamash) May 18, 2019

I offer these conclusions only after having read Mueller’s redacted report carefully and completely, having read or watched pertinent statements and testimony, and having discussed this matter with my staff, who thoroughly reviewed materials and provided me with further analysis. — Justin Amash (@justinamash) May 18, 2019

In comparing Barr’s principal conclusions, congressional testimony, and other statements to Mueller’s report, it is clear that Barr intended to mislead the public about Special Counsel Robert Mueller’s analysis and findings. — Justin Amash (@justinamash) May 18, 2019

Barr’s misrepresentations are significant but often subtle, frequently taking the form of sleight-of-hand qualifications or logical fallacies, which he hopes people will not notice. — Justin Amash (@justinamash) May 18, 2019

Under our Constitution, the president “shall be removed from Office on Impeachment for, and Conviction of, Treason, Bribery, or other high Crimes and Misdemeanors.” While “high Crimes and Misdemeanors” is not defined, the context implies conduct that violates the public trust. — Justin Amash (@justinamash) May 18, 2019

Contrary to Barr’s portrayal, Mueller’s report reveals that President Trump engaged in specific actions and a pattern of behavior that meet the threshold for impeachment. — Justin Amash (@justinamash) May 18, 2019

In fact, Mueller’s report identifies multiple examples of conduct satisfying all the elements of obstruction of justice, and undoubtedly any person who is not the president of the United States would be indicted based on such evidence. — Justin Amash (@justinamash) May 18, 2019

Impeachment, which is a special form of indictment, does not even require probable cause that a crime (e.g., obstruction of justice) has been committed; it simply requires a finding that an official has engaged in careless, abusive, corrupt, or otherwise dishonorable conduct. — Justin Amash (@justinamash) May 18, 2019

While impeachment should be undertaken only in extraordinary circumstances, the risk we face in an environment of extreme partisanship is not that Congress will employ it as a remedy too often but rather that Congress will employ it so rarely that it cannot deter misconduct. — Justin Amash (@justinamash) May 18, 2019

Our system of checks and balances relies on each branch’s jealously guarding its powers and upholding its duties under our Constitution. When loyalty to a political party or to an individual trumps loyalty to the Constitution, the Rule of Law—the foundation of liberty—crumbles. — Justin Amash (@justinamash) May 18, 2019

We’ve witnessed members of Congress from both parties shift their views 180 degrees—on the importance of character, on the principles of obstruction of justice—depending on whether they’re discussing Bill Clinton or Donald Trump. — Justin Amash (@justinamash) May 18, 2019

Few members of Congress even read Mueller’s report; their minds were made up based on partisan affiliation—and it showed, with representatives and senators from both parties issuing definitive statements on the 448-page report’s conclusions within just hours of its release. — Justin Amash (@justinamash) May 18, 2019