Rep. Justin Amash, I-Mich., is seen on the House steps of the Capitol before the House passed a $2 trillion coronavirus aid package by voice vote on Friday, March 27, 2020.

Michigan Rep. Justin Amash may be an outspoken conservative from a Republican-leaning district, but the launch of his exploratory committee seeking a presidential run was met with swift backlash from supporters of apparent Democratic nominee Joe Biden.

Critics say a third-party bid by Amash, an independent who endeared himself to many Democrats by quitting the Republican Party and voting to impeach President Donald Trump, would likely siphon votes away from Biden in the 2020 election.

"I like Justin. I love democracy. But I do think a 3rd party run increases the chances of Trump's re-election," tweeted Andrew Yang, a former Democratic presidential primary candidate who has endorsed Biden.

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Trump appeared to agree.

"I think Amash would make a wonderful candidate," Trump said in a derisive tweet Wednesday, "especially since he is way behind in his district and has no chance of maintaining his Congressional seat."

"He almost always votes for the Do Nothing Dems anyway. I like him even more than Jill Stein!" Trump added, referring to the former Green Party nominee who won about 1% of the popular vote in the 2016 election, mainly from liberal states.

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Amash, who would run on the Libertarian Party ticket, denied that his entree in the race would necessarily take votes away from Biden.

"We don't know who people will vote for. It's impossible to say whether more people will vote for Biden or Trump if I'm in the race or not in the race," Amash said Wednesday morning on MSNBC. "So I think there's a big factual issue there."

Amash also pushed back on the notion that Americans have to vote in lockstep with their political parties.

"The way we got Donald Trump is because every Republican who didn't like Donald Trump was told ... you have to vote for the Republican. Similarly, people are being told today on the Democratic side, you have to vote for Joe Biden and Republicans are being told you have to vote for Donald Trump again," Amash said. "That's not the way the system is supposed to work."

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But even some of Amash's self-professed fans call his move "perplexing."

Joe Walsh, a former Illinois congressman who had mounted a now-defunct Republican primary challenge against Trump, wrote in a Washington Post op-ed that Amash running for president on a Libertarian ticket is "a terrible idea."

"Amash can't win. But he can siphon enough votes from the Democratic nominee, Joe Biden, to hand the election to Trump," Walsh wrote.

George Conway, a conservative Trump critic and husband of White House advisor Kellyanne Conway, tweeted: "Needless to say, my views align more closely with Amash's than Biden's. But the only real effect Amash could have in this campaign is to enhance Trump's chances. This is a terrible idea."

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Amash tweeted later Wednesday that "the visceral outrage" that followed his announcement "speaks volumes about the ugly, hyperpartisan nature of politics today."

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Michigan-based GOP consultant Saul Anuzis told CNBC that Trump's opponents may be more hostile to Amash because of the type of campaign Biden is mounting.

"If you're running an anti-Trump campaign, you're trying to rally all the forces," making it problematic when a potentially viable protest vote emerges, Anuzis said.

"When the Republican who walks into the polls says, 'I really don't like Trump, but I hate to see what Biden would do instead,' they can still cast their protest vote for Amash," Anuzis said. "That's why Democrats are more fearful."

The 40-year-old congressman, now on his fifth term in the House — and his first since leaving the GOP in July 2019 — has voted with Trump more than 63% of the time, according to stat blog FiveThirtyEight.

His votes against key Democratic bills such as the Affordable Care Act, and his more recent opposition to funding packages providing economic relief amid the coronavirus crisis, have won him few liberal fans.

But Amash drew national media attention and waves of praise from Democrats last year, when he forcefully argued that former special counsel Robert Mueller's report on Russian election meddling showed that Trump had "engaged in impeachable conduct."

Amash was still a Republican at the time.

The special counsel had investigated Russian election interference, as well as possible coordination between the Trump campaign and the Kremlin and possible obstruction of justice by Trump. Mueller said his probe did not uncover evidence of coordination between Russia and the campaign but highlighted multiple instances of possible obstruction of justice by Trump.

In December, Amash voted to impeach Trump on articles of obstruction of Congress and abuse of power, related to the president's efforts to have Ukraine announce investigations into his Democratic rivals, including Biden.

"President Donald J. Trump has abused and violated the public trust by using his high office to solicit the aid of a foreign power, not for the benefit of the United States of America but instead for his personal and political gain," Amash said before casting his vote.

The Democrat-led House passed those articles of impeachment along nearly partisan lines. The GOP-majority Senate voted to acquit Trump on both charges.

Amash is facing a tough reelection fight in his Michigan district, which in 2016 had voted for Trump over Democratic nominee Hillary Clinton by nearly 10 percentage points.

His new political affiliation and his frequent attacks against Trump have soured his reputation among establishment Republicans and the groups that typically back GOP candidates. Organizations such as the conservative Club for Growth, one of Amash's largest backers in past elections, have said they would not support his reelection bid.

Amash told Reason that he believed he would win his House race, but "I just think this is too important."

He faces both Republican and Democratic challengers in the 3rd District's primary but he has a fundraising advantage despite losing some backers.

Even so, Anuzis told CNBC that the congressional race "absolutely" played a role in Amash's decision to explore a White House bid.

"He has a very good chance of losing" in the district, Anuzis said. "If he's going to go down, he might as well go down running for president."