Tea Party firebrand and former GOP Rep. Joe Walsh announced Sunday that he will run against President Donald Trump in the 2020 Republican primary.

" We've got a guy in the White House who is unfit, completely unfit to be president and it stuns me that nobody stepped up, nobody in the Republican party stepped up... everybody believes in the Republican party, everybody believes he's unfit," Walsh told ABC's George Stephanopoulos. "He lies every time he opens his mouth."

Walsh was an early supporter of Trump but quickly evolved into one of his sharpest critics on the right.

In recent days, there's been speculation that he wants to recruit George Conway, the husband of White House counselor Kellyanne Conway and another fierce Trump critic, for a senior role on his campaign.

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Joe Walsh, the former Republican congressman from Illinois and a Tea Party firebrand, announced Sunday that he will run against President Donald Trump in the 2020 Republican primary.

"We've got a guy in the White House who is unfit, completely unfit to be president and it stuns me that nobody stepped up, nobody in the Republican party stepped up, because I'll tell you what George, everybody believes in the Republican party, everybody believes he's unfit," Walsh told ABC's George Stephanopoulos on "This Week." "He lies every time he opens his mouth."

Walsh also confirmed the news on his Twitter, writing, "Friends, I'm in. We can't take four more years of Donald Trump. And that's why I'm running for President."

"It won't be easy, but bravery is never easy," he added. "But together, we can do it." Walsh also included a link to his official website and urged supporters to donate to his campaign.

Walsh served in Congress from 2011 to 2013, and while he has little support from the GOP establishment, he enjoys broad approval within the ultra-conservative Tea Party movement.

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Walsh has a history of making controversial statements, particularly on his right-wing talk radio show. In 2014, he was pulled off from his show for one day for using racial slurs on the air. He said in a statement that he'd been kicked off for "trying to have an honest, adult conversation about words without resorting to alphabet soup phrases (C-word, N-word, etc)."

During the 2016 presidential race, Walsh was sharply criticized for posting a tweet that some interpreted as a call for violence against Black Lives Matter and other activists protesting police brutality. After the shooting of five Dallas police officers on July 7, 2016, Walsh tweeted, "This is now war. Watch out Obama. Watch out Black Lives Matter punks. Real America is coming for you."

Walsh later wrote that he "wasn't calling for violence, against Obama or anyone. Obama's words and BLM's deeds have gotten cops killed. Time for us to defend our cops."

Walsh was an early supporter of Trump, and although he later soured on Trump throughout the 2016 election, Walsh still voted for him against the Democratic nominee, Hillary Clinton. But Walsh evolved into a frequent and harsh Trump critic after the election. Walsh infuriated other conservatives when he said he supported the former special counsel Robert Mueller and the FBI's investigation into Russia's interference in the 2016 election.

Walsh's fiercest criticism of Trump came after the president's summit in Helsinki with Russian president Vladimir Putin, during which Trump sided with Putin over the US intelligence community and suggested the FBI and Democrats were conspiring against him.

"I will never support Trump again," Walsh said at the time, adding that Trump is "a danger to this country."

Walsh also criticized Trump for promoting violence against journalists through his praise for Montana Rep. Greg Gianforte, who made headlines for body-slamming a reporter while campaigning for his seat in Congress.

There's been speculation in recent days that Walsh wants to recruit George Conway, the husband of White House counselor Kellyanne Conway and a frequent Trump critic, for a senior position on his campaign.

"I think Walsh's plan to attack Trump for his dishonesty, amorality, instability and incompetence is absolutely the right approach, and I'll do whatever I can to help," Conway told The New York Times. He declined to say whether he would formally join the campaign.