Rep. Justin Amash Justin AmashInternal Democratic poll shows tight race in contest to replace Amash Centrist Democrats 'strongly considering' discharge petition on GOP PPP bill On The Trail: How Nancy Pelosi could improbably become president MORE (Mich.) doubled down on both his support for impeaching President Trump Donald John TrumpSteele Dossier sub-source was subject of FBI counterintelligence probe Pelosi slams Trump executive order on pre-existing conditions: It 'isn't worth the paper it's signed on' Trump 'no longer angry' at Romney because of Supreme Court stance MORE and his decision to leave the Republican party on CNN’s “State of the Union” Sunday.

Amash, who had previously been the only Republican member of Congress to support impeachment proceedings, told CNN’s Jake Tapper Jacob (Jake) Paul TapperThe media's misleading use of COVID-19 data Julia Louis-Dreyfus: 'We can't spend much time grieving' Ginsburg Pence aide dismisses concerns rushed vote on Trump nominee will hurt vulnerable senators MORE on Sunday that senior officials have privately thanked him for his public stance.

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"When I was discussing impeachment, I had fellow colleagues and other Republicans, high-level officials, contacting me, saying, 'Thank you for what you’re doing,'" Amash said. "So there are lots of Republicans out there who are saying these things privately, but they're not saying it publicly, and I think that’s a problem for our country."

Amash also declined again to rule out a third-party presidential run.

"I still wouldn't rule anything like that out. I believe that I have to use my skills, my public influence, where it serves the country best. And I believe I have to defend the Constitution in whichever way works best," he said.

Trump, he added, "thinks people owe loyalty to him, but people are elected to Congress with an oath to support and defend the Constitution, not an oath to support and defend one person, the president who happens to be from your own party."

Amash criticized Speaker Nancy Pelosi Nancy PelosiPelosi slams Trump executive order on pre-existing conditions: It 'isn't worth the paper it's signed on' On The Money: Anxious Democrats push for vote on COVID-19 aid | Pelosi, Mnuchin ready to restart talks | Weekly jobless claims increase | Senate treads close to shutdown deadline Trump signs largely symbolic pre-existing conditions order amid lawsuit MORE (D-Calif.) for her continued hard line against pursuing impeachment.

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

"I do believe there's a strong case. I believe she believes there is a strong case, and if so, she should move forward," he added.

The four-term lawmaker has said he arrived at his position on impeachment after reading former special counsel Robert Mueller Robert (Bob) MuellerCNN's Toobin warns McCabe is in 'perilous condition' with emboldened Trump CNN anchor rips Trump over Stone while evoking Clinton-Lynch tarmac meeting The Hill's 12:30 Report: New Hampshire fallout MORE’s report, in which which Mueller determined he could not exonerate Trump on the question of obstruction of justice but did not find sufficient evidence the Trump campaign coordinated or conspired with the Russian government on election interference.

Amash told Tapper the majority of Americans likely have not read the report and are counting on their elected representatives to do so, but he added that he believes less than 15 percent of Congress has read it.

If all Congressional Republicans read the report, Amash said, "I think a large number of them would reach the same conclusion. There are some that who would reach different conclusions."

The Sunday appearance followed a Thursday op-ed by Amash in The Washington Post in which he announced his departure from the GOP.

"Today, I am declaring my independence and leaving the Republican Party," Amash wrote on the Fourth of July. "No matter your circumstance, I’m asking you to join me in rejecting the partisan loyalties and rhetoric that divide and dehumanize us. I’m asking you to believe that we can do better than this two-party system — and to work toward it. If we continue to take America for granted, we will lose it."

His stance on impeachment has resulted in sharp backlash from his Republican colleagues and supporters of the president. At his first town hall since coming out in favor of impeachment, he received a standing ovation but also took a question from a constituent in a "MAGA" hat who said, "I can’t tell you how disappointed I am."

House Minority Leader Kevin McCarthy Kevin Owen McCarthyMcCarthy says there will be a peaceful transition if Biden wins GOP lawmakers distance themselves from Trump comments on transfer of power McCarthy claims protests in Louisville, other cities are 'planned, orchestrated events' MORE (R-Calif.) said in May that Amash was "out of step with this conference” and “out of step with America."

"You know, people get elected by going back and talking to the constituents of how they vote on the floor, a reflection of within their own district," he said at a press conference. "I think when it comes to actions, I think the constituents will have the final say."

Amash attracted a primary challenger almost immediately after announcing his support for impeachment. Michigan state Rep. Jim Lower entered the race touting his support for Trump and opposition to impeachment, and Trump is reportedly considering publicly backing him.

"If they were to get involved early I think it would help make sure that it was just me versus Amash in the August primary — and if that’s the case we’re definitely going to win," Lower told Politico in June.

Donald Trump Jr. Don John Trump'Tiger King' star Joe Exotic requests pardon from Trump: 'Be my hero please' Zaid Jilani discusses Trump's move to cancel racial sensitivity training at federal agencies Trump International Hotel in Vancouver closes permanently MORE has also hinted he will campaign with Lower, with Amash countering, “If it’s what you say I love it, especially later in the summer,” a reference to an email the president’s eldest son sent while planning a 2016 meeting at Trump Tower with Russian lawyer Natalia Veselnitskaya.