"That letter selectively quotes and summarizes points in Mueller's report in misleading ways", Amash wrote, adding that Barr "in subsequent statements and testimony ... used further misrepresentations to help build the president's false narrative that the investigation was unjustified".
Amash, a self-identifying libertarian Republican, pushed back on the accusation. In recent days, Amash has also tweeted criticism of the president's approval of arms sales to Saudi Arabia and his use of his pardon power.
"I don't believe you can criminally indict a sitting president - I think that's what the founders believed", Amash said during a town hall event in Grand Rapids on Tuesday.
On Tuesday, Amash said his Republican colleagues had privately spoken to him and said, "Justin, you know, going out publicly with that, you know the Democrats will never support you, you know that they're hypocrites on this stuff".
"Congress has a constitutional obligation to conduct oversight of the executive branch", he said in a statement.
"I'll let the president make that determination", she said.
Amash said, "I think it's really important that we do our job as a Congress, that we not allow misconduct to go undeterred, that we not just say, someone can violate the public trust and that there are no consequences to it".
The only Republican in Congress to accuse President Trump of impeachable conduct faced voters in his MI district Tuesday, receiving standing ovations from many who saluted his "courage" while sparring with some former supporters who faulted him for embracing a Democratic "smear attack" against Trump.
Republicans, as they have done since Mueller's report was released, called for Congress to move on.
This is not the first time Amash has not fallen along party lines, as he has also disagreed with his party in the past when it came to certain foreign policy and surveillance issues. Last week he joined the chorus of Democrats calling for President Trump's impeachment. In response, the DeVos family, a powerful entity in Conservative politics, announced it would stop supporting Amash.
Amash is expected to face Trump-backed challenger Jim Lower in the 2020 primaries.
House Judiciary Chair Jerry Nadler said Mueller's remarks - which broke no new ground - still left open the question of whether Team Trump obstructed justice.