Washington (CNN) Newly independent Rep. Justin Amash said in an interview Sunday with CNN that House Speaker Nancy Pelosi is "making a mistake" in not beginning impeachment proceedings against President Donald Trump.

Amash was the only congressional Republican to publicly argue that the President has engaged in impeachable conduct before announcing he was leaving the Republican Party on Thursday. He told CNN's Jake Tapper on "State of the Union" Sunday that the California Democrat needs to be clear in her message and if she believes there is impeachable conduct, she should say so.

"From a principled, moral position, she's making a mistake. From a strategic position, she's making a mistake," Amash said. "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."

Amash 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."

Pelosi told CNN's Manu Raju in an interview last month that moving forward on impeachment proceedings against Trump is "not off the table," but that she continues to oppose formally opening the process.

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