The Michigan representative set out the case with a lengthy Twitter thread, arguing Mr Trump had “engaged in specific actions and a pattern of behaviour that meet the threshold for impeachment”.
Mr Amash said the Mueller report into Russian election inference and related matters “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”.
The congressman also attacked William Barr, saying “it is clear” the attorney general intended to mislead the public about the Mueller report in both his written conclusions and congressional testimony.
“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.
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
A frequent critic of the president, Mr Amash previously signalled he would consider running as a libertarian against Trump in the 2020 election. In February he became the only Republican to co-sponsor a resolution in the US House of Representatives to reject the emergency Mr Trump declared at the US-Mexico border to build a wall there.
Neither the White House nor the Department of Justice have responded to his tweets on impeachment. Ronna McDaniel, the chairwoman of the Republican National Committee said “it’s sad to see ... Amash parroting the Democrats’ talking points on Russia”.
She said the only people still concerned about the Russia investigation are Mr Trump’s political foes “hoping to defeat him in 2020 by any desperate means possible”.
Mr Amash, who represents Michigan’s 3rd congressional district, wrote that he had read the full Mueller report, but that few members of Congress had. He claimed “their minds were made up based in partisan affiliation”.
The Republican said impeachment should be undertaken only in extraordinary circumstances, but added: “America’s institutions depend on officials to uphold both the rules and spirit of our constitutional system even when to do so is personally inconvenient or yields a politically unfavourable outcome.”
Mr Trump has repeatedly claimed Mr Mueller’s report concluded there no obstruction of justice, yet the report made no formal finding on that question, leaving it up to Congress.
In his letter to Congress, Mr Barr said he and his deputy Rod Rosenstein determined there was insufficient evidence to establish that the president committed criminal obstruction of justice, or acted unlawfully to impede the investigation.
House speaker Nancy Pelosi said earlier this month that Mr Trump was moving closer to impeachment with his efforts to thwart lawmakers’ efforts to oversee his administration.
She said the president was becoming “self-impeachable” due to his administration’s repeated obstruction of congressional investigations.
Democrats are divided about impeachment, however, and Ms Pelosi has also said impeachment proceedings would be “divisive” for the country.