"I'm going to run for president," Walsh told Stephanopoulos in an exclusive interview.
When Stephanopoulos pointed out the massive uphill climb Walsh has in front of him in the primary thanks to Trump's overwhelmingly high approval rating within the party, the controversial former congressman argued that conservatives should have an alternative to the president.
"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," said Walsh, who was elected to the House in the 2010 Tea Party wave, but only served one term before becoming a conservative talk radio host.
Trump campaign communications director Tim Murtaugh sent ABC News a dismissive one-word response to Walsh jumping into the race: "Whatever."
Republican National Committee Chair Ronna McDaniel told ABC News in response to Walsh's announcement, "President Trump enjoys unprecedented support among Republicans. He has already delivered a long list of incredible accomplishments for conservatives and the country. Republicans are firmly behind the president and any effort to challenge him in a primary is bound to go absolutely nowhere."
But while Walsh has argued he plans to make a moral case against the president, Stephanopoulos asked the former congressman if he's the best person to make that argument given his long history of incendiary and controversial statements ranging from using racist slurs on Twitter to promoting falsehoods around former President Barack Obama's birth certificate and religion.
"I helped create Trump, and George, that's not an easy thing to say," Walsh told Stephanopoulos. "I went beyond the policy and the idea differences and I got personal and I got hateful. I said some ugly things about President Obama that I regret."
Stephanopoulos pushed back and pointed out a number of Walsh's comments, including calling Obama a Muslim, an enemy and a traitor, and as recently as August 2017 tweeting, "Senator Kamala Harris said something really dumb. Meh. If you're black and a woman, you can say dumb things. Lowered bar."
"That is kind of textbook racism and sexism," Stephanopoulos told Walsh. The former congressman responded by saying Trump's time in office has made him "reflect on some of the things I have said in the past."
"Did you really believe [Obama is] a Muslim?," Stephanopoulos asked.
"God no. And I have apologized for that," Walsh said. "I'm baring my soul with you right now on national TV. We have a guy in the White House who's never apologized for anything he's done or said."
When asked, Walsh also told ABC News' chief anchor that invoking the 25th Amendment to remove Trump from office for being unfit should be "looked at" because "we've never had a situation like this. You can't believe a word he says."
Throughout Sunday's interview, Walsh used harsh, inflammatory language to describe the president — calling him "incompetent," "nuts," "erratic," "narcissist," "bully," "coward," "completely unfit," "disloyal," and "un-American."
Stephanopoulos posed to Walsh that in the past, presidents who've faced serious primary challengers tend to be weakened and lose in the general election.
"Are you prepared to take responsibility, if you do well in this, for helping elected a Democrat that some of your viewers and listeners will say, 'Oh, socialist?'" Stephanopoulos asked.
"It doesn't matter. Absolutely, I'm going to do whatever I can," Walsh said on "This Week." "I don't want him to win. The country cannot afford to have him win. If I'm not successful, I'm not voting for him."
Walsh said his campaign plans to focus on New Hampshire and Iowa and will be on TV as much as he can with the hopes of his long-shot bid catching on like wildfire. "And if you're wrong?" Stephanopoulos asked.
"If I'm wrong, it was the right fight, because somebody had to do this," Walsh said in response.
The former Illinois congressman-turned-radio host was once a fervent Trump supporter who's become a fierce critic of the president. Walsh is just the second Republican to jump into the primary behind former Massachusetts Gov. Bill Weld, who announced back in April but has yet to gain serious traction.
Walsh has acknowledged there's little chance his candidacy will result in Trump losing the party nomination, and he is instead focused on offering GOP voters an alternative vision for the party.
Walsh only served one term in Congress, but his candidacy does perhaps bring a more current figure from conservative circles into the long-shot picture compared to Weld, who last held public office over 20 years ago. Walsh's nationally syndicated radio show and large online following arguably kept him more relevant.
The Trump administration's latest actions around trade and the economy along with what Walsh's team calls an "incredible reaction" and flood of support to an op-ed Walsh published in the New York Times last week is what pushed the conservative radio host to jump into the race.
Walsh's team says he is set to travel to and spend "a lot of time" in the key primary states of Iowa and New Hampshire in the coming weeks.