House speaker Nancy Pelosi is making a moral and strategic mistake by refusing to bring formal articles of impeachment against Donald Trump, the only Republican in Congress to back such proceedings said on Sunday.

The Michigan representative Justin Amash is in fact now an ex-Republican, having announced on 4 July that he is quitting the party to become an independent and fight the “partisan death spiral” of modern US politics.

In an interview on CNN’s State of the Union, he did not rule out a rumoured run for the White House.

“I believe that I have to use my skills, my public influence, where it serves the country best,” he said. “And I believe I have to defend the constitution in whatever way works best.”

Such a campaign from the right could complicate Trump’s hopes of re-election in 2020. On CNN, Amash defended his role as a founder of the hard-right Freedom Caucus, a key support bloc for the president on which many place much of the blame for deadlock in Congress.

Amash also said he believed he could win re-election as an independent.

He first called for impeachment – and duly earned stringent abuse from the president and attacks from others in the party – in May, while still a Republican.

Amash bases his case for an attempt to remove Trump on the report of special counsel Robert Mueller and his own oft-professed dedication to the meaning and spirit of the US constitution.

Mueller, a former FBI director, did not find evidence of conspiracy between Trump and Russia during the 2016 election, in which Russia interfered with the aim of electing Trump over the Democratic nominee, Hillary Clinton.

But he did lay out extensive contacts between the Trump campaign and Russians and detailed 11 instances of possible obstruction of justice by the president or his aides.

Mueller explicitly said he was not exonerating Trump but in a widely criticised move, attorney general William Barr said the president had no case to answer. Trump has repeatedly claimed vindication.

Democrats and many constitutional scholars contend that in his report, and in subsequent remarks to the press, Mueller was laying out a “road map” to impeachment. His testimony before the House judiciary and intelligence committees on 17 July is eagerly awaited.

Pelosi has declined to move on growing calls to move impeachment forward. To some, this seems sound political judgment. Impeachment would be unlikely to succeed in the Senate and could galvanise Trump’s base ahead of the 2020 election.

To others, it seems a dereliction of constitutional duty.

On Sunday, Amash said he thought that “from a principled, moral position, [Pelosi] is making a mistake. From a strategic position, she’s making a mistake.

“If she believes, as I do, that there’s impeachable conduct in there, then she should say so. She should tell the American people, ‘We’re going to move forward with impeachment hearings and potentially articles of impeachment.’”

He continued: “When she says things like, ‘Oh, I think that we need to have the strongest case before we go forward,’ what she’s telling the American people is, she doesn’t think there’s a strong case. If she doesn’t think that, then she shouldn’t open her mouth in the first place and say she thinks there’s impeachable conduct.

“I do believe there’s a strong case. I believe she believes there’s a strong case. And, if so, she should move forward and make sure that the American people understand what’s going on, because people at home aren’t reading the Mueller report.

“Most people don’t have time to read a 448-page report. They expect their members of Congress to do the work for them. They want speaker Pelosi to do the work. They want other members to do the work. And if she doesn’t want to go forward, then we’re going to have a big problem.”

Amash was also asked how many Republicans he thought had read the Mueller report, which was published with redactions.

“I think it’s probably less than 15%,” he said. “And I would say that’s probably the case on both sides of the aisle.”

If members of Congress did read it, he said, he thought “a large number of them would reach the same conclusion.

“When you look at the conduct in there, when you look at the evidence that’s presented, I think basically anyone would be indicted for that conduct, anyone who is not the president of the United States.”