Congressman Justin Amash from Michigan, a frequent critic of U.S. President Donald Trump, on Saturday became the first Republican lawmaker to say the president has engaged in impeachable behavior. His fellow Michigan representative, Rashida Tlaib, followed up on Amash's remarkable statement by urging him to sign on to a resolution introducing articles of impeachment against Trump.

Tlaib has called for Trump's impeachment since taking office in January and both she and Amash share Palestinian roots.

Special Counsel Robert Mueller's report on the investigation into Russian interference in the 2016 election reveals that Trump "engaged in specific actions and a pattern of behavior that meet the threshold for impeachment," Amash, who has signaled he would consider running as a libertarian against Trump in the 2020 election, wrote on Twitter.

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," Amash wrote.

Trump responded to Amash's statements on Sunday, writing, "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,....

....he would see that it was nevertheless strong on NO COLLUSION and, ultimately, NO OBSTRUCTION...Anyway, how do you Obstruct when there is no crime and, in fact, the crimes were committed by the other side? Justin is a loser who sadly plays right into our opponents hands!"

Trump has said Mueller's report concluded there no obstruction of justice. Mueller's report made no formal finding on that question, leaving the matter up to Congress.

Amash also wrote that "it is clear" that Attorney General William Barr intended to mislead the public about Mueller's report in his conclusions and congressional testimony about it.

In his letter to Congress, 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.

Amash's comments echoed the conclusions of many Democrats. House Speaker Nancy Pelosi said on May 8 that Trump was moving closer to impeachment with his efforts to thwart congressional subpoenas and obstruct lawmakers' efforts to oversee his administration.

Still, Democrats are divided about impeachment and Pelosi also said impeachment proceedings would be "divisive" for the country.

The White House and the Department of Justice did not immediately respond to requests for comments about Amash's tweets.

Ronna McDaniel, the chairwoman of the Republican National Committee wrote on Twitter "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 Trump's political foes "hoping to defeat him in 2020 by any desperate means possible."

Amash, who represents Michigan's 3rd congressional district, wrote that he had read Mueller's full redacted report, but that few members of Congress had.

In February Amash became the lone Republican to co-sponsor a resolution in the U.S. House of Representatives to reject the emergency Trump declared at the U.S.-Mexico border to build a wall there, in a stinging rebuke to the president..

Impeachment should be undertaken only in extraordinary circumstances, Amash wrote on Saturday. But the risk during a time 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."