Former Rep. Joe Walsh Joe WalshSunday shows preview: Protests continue over shooting of Blake; coronavirus legislation talks remain at impasse Republicans officially renominate Trump for president Tucker Carlson responds to guest correcting pronunciation of Kamala Harris's name: 'So what?' MORE (R-Ill.) on Sunday announced he would mount a primary challenge to President Trump Donald John TrumpFederal prosecutor speaks out, says Barr 'has brought shame' on Justice Dept. Former Pence aide: White House staffers discussed Trump refusing to leave office Progressive group buys domain name of Trump's No. 1 Supreme Court pick MORE.

The conservative radio host unveiled his campaign on ABC's "This Week."

"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 that he's unfit," Walsh told ABC's George Stephanopoulos George Robert StephanopoulosColbert implores Pelosi to update 'weaponry' in SCOTUS fight: 'Trump has a literal heat ray' Murkowski: Supreme Court nominee should not be taken up before election Cruz says Senate Republicans likely have votes to confirm Trump Supreme Court nominee MORE. "He lies every time he opens his mouth."

"I'm running because he's unfit, somebody needs to step up and there needs to be an alternative. The country is sick of this guy's tantrum, he's a child," Joe Walsh tells @GStephanopoulos when asked about GOP support for Trump https://t.co/bLWbG7aCAU pic.twitter.com/KeFCo7sJmL — This Week (@ThisWeekABC) August 25, 2019

Walsh was elected to the House in 2010 as part of the Tea Party wave and served one term. He began as a strong supporter of President Trump but over the past year has emerged as a fierce critic of the president. On Sunday, he apologized for tweets Stephanopoulos described as racist and sexist as well as his "personal" criticism of former President Obama, acknowledging the role it played in creating today's politics.

"I helped create Trump. There's not doubt about that. The personal, ugly politics," he said. "I regret that. And I'm sorry for that. And now we've got a guy in the White House, that's all he does."

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When asked about Trump's high approval rating among Republicans, Walsh said GOP voters had no alternative, adding that many of his former colleagues in the House feel the same way.

"I'm running because he's unfit. Somebody needs to step up, and there needs to be an alternative," Walsh said. "The country is sick of this guy's tantrum. He's a child. Again, the litany. He lies every time he opens his mouth. Look at what's happened this week. The president of the United States is tweeting us into a recession."

After being asked about invoking the 25th Amendment to remove Trump from office, he added, "We've never had a situation like this. ... You can't believe a word he says. He's nuts. He's erratic. He's cruel. He stokes bigotry. He's incompetent. He doesn't know what he's doing. George, he's a narcissist. The only thing he cares about is Trump."

Walsh in a tweet announcing his campaign said the U.S. "can't take four more years of Donald Trump."

"It won't be easy, but bravery is never easy," he added.

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. But together, we can do it. Join me... join us: go to https://t.co/d40HA9h2Kz.



Let's show the world we're ready to be brave. — Joe Walsh (@WalshFreedom) August 25, 2019

This breaking news report was last updated at 9:43 a.m.