This is not the first time Mr. Amash, who considers himself a libertarian and strict constitutionalist, has defied the White House and bucked his party’s Capitol Hill leadership.
Mr. Amash was one of 14 Republicans to side with Democrats in their unsuccessful attempt to override the president’s first veto, which upheld an emergency declaration to divert funding from other federal projects to build a wall along the southwestern border.
Republican leaders in both chambers have griped privately about Mr. Trump’s behavior but have publicly fallen in line behind the president. Mr. Trump has seized on Mr. Mueller’s finding that his campaign did not coordinate with Russia to declare that the investigation was an exoneration. (In fact, the report stopped short of clearing the president on the question of whether he illegally obstructed justice.)
On Thursday, Ms. Pelosi said she would consider impeachment only as a means of compelling Mr. Barr and Donald F. McGahn II, the former White House counsel who cooperated extensively with the special counsel’s inquiry, to testify — despite Mr. Trump’s blanket call for current and former aides to stonewall all requests by Democrats for testimony or documents.
But first, she added, House committees must exhaust all their legal and legislative options.
“First we ask, then we subpoena friendly,” she said. “Then we subpoena otherwise, and then we see what we get. So let’s not leapfrog over what we think should be the path that should be taken.”
Few Republicans, especially House members like Mr. Amash who face re-election every two years, have been willing to confront Mr. Trump publicly. Former Representative Mark Sanford of South Carolina has repeatedly criticized the president and questioned his conduct — and his party’s acquiescence to him.
“The idea of some allegiance not to the Constitution, but to the president, was not what I signed up for,” Mr. Sanford told The Washington Post last year.
Over the summer, Mr. Sanford lost the Republican primary race to a Trump ally, Katie Arrington, hours after Mr. Trump called him “very unhelpful to me in my campaign to MAGA.”