The Michigan congressman Justin Amash has called for Donald Trump to be impeached, adding a first Republican voice to a growing chorus of Democrats demanding Congress move against the president.

Amash is a libertarian and independent-minded politician who has flirted with the idea of a run against Trump in 2020, and has in turn been attacked by the White House. Elected in the Tea Party wave of 2010, he was a founder of the House Freedom Caucus, which has become a hard-right mouthpiece for Trump.

No other Republican in Congress has said Trump should be impeached.

In a series of tweets on Saturday, Amash said special counsel Robert Mueller had in his investigation of Russian election interference identified “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”.

Mueller did not find evidence of a conspiracy between Trump and Russia but did lay out extensive contacts between aides and Moscow and 11 instances of potential obstruction of justice by the president or his campaign. On the obstruction question, Mueller said Congress should decide what happened next.

In summarising Mueller’s work before Congress had seen it, however, the attorney general, William Barr, said he had decided Trump had not obstructed justice. The president and his supporters immediately claimed total exoneration, a drumbeat that has not slackened since.

Amash said Barr “has deliberately misrepresented” Mueller’s work in presentations to the public and congressional testimony.

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

Of that threshold, he wrote that though the “high crimes and misdemeanors” mentioned in the constitution are “not defined, the context implies conduct that violates the public trust”.

Impeachment, he wrote, “simply requires a finding that an official has engaged in careless, abusive, corrupt or otherwise dishonorable conduct”.

Impeachment would be initiated by the House judiciary committee. But in a political calculation Democratic leaders have held off, unsure of the effect on voters and sure the process would fail in the Republican-controlled Senate, where a two-thirds majority would be required to convict Trump and remove him from office.

Amash lamented the effect of America’s political divide, writing that “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.

“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.”

The Trump administration has refused to comply with House requests and subpoenas for records including the un-redacted Mueller report and its underlying evidence and Trump’s tax returns, raising claims on the left that the president is acting against the constitution in a dangerous grab for power.

Amash said that while “few members of Congress even read Mueller’s report” – he said he had, in its entirety, and had consulted with his staff – “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.”

Democrats and Republicans had “shift[ed] their views 180 degrees”, he said, “depending on whether they’re discussing Bill Clinton or Donald Trump”.

Twenty years ago, Clinton was impeached by a Republican House but acquitted by the Senate. Senator Lindsey Graham of South Carolina, a key Trump supporter now, was a House impeachment manager then. His statements from two decades ago have been played in near-rotation on mainstream cable news.

Amash concluded by saying elected officials should “uphold both the rules and spirit of our constitutional system even when to do so is personally inconvenient or yields a politically unfavorable outcome”.