WASHINGTON — Joe Walsh — the ex-suburban Chicago GOP House member turned conservative talk show host who once backed President Donald Trump — on Sunday said he will challenge Trump in the GOP primary.

“I’m going to run for president. I’m going to challenge this guy,” Walsh told ABC “This Week” host George George Stephanopoulos.

His slogan in his very long shot 2020 bid is “Be Brave,” Walsh said.

“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,” Walsh said.

Trump campaign director of communications Tim Murtaugh shrugged off Walsh’s entry into the primary in an email to the Sun-Times; when asked for a comment, Murtaugh responded: “Whatever.”

A Mundelein resident, Walsh, 57, was elected on the 2010 Tea Party wave to Congress, serving one term. He became a conservative talk-radio show host after he lost his House seat to Tammy Duckworth, now the junior U.S. senator from Illinois.

On ABC, Walsh also said of Trump, “We’ve never had a situation like this, you can’t believe a word he says ... he’s nuts ... the only thing he cares about is Trump.”

Walsh, no stranger to controversy and incendiary comments, said he knows he is “opening up my life” by taking on Trump, running from the right and making the moral argument that Trump is unfit for office.

“I’m going to pound Trump every single day.”

At about the same time on Sunday that Walsh made his announcement on TV, he said in a tweet, “Friends, I’m in. We can’t take four more years of Donald Trump. And that’s why I’m running for President.”

Trump has the support of state and local Republican leaders and controls the Republican National Committee and the major fundraising committees.

Walsh also released a video announcing his candidacy on his website, noting that he is skipping an exploratory, testing-the-waters pre-announcement phase.

“So the hell with all those conventional things,” Walsh said in his video. “It’s time to be brave. We have someone in the White House who we all know is unfit, someone who lies virtually every time he opens his mouth.”

Former Massachusetts Gov. William Weld has also said he’ll run against Trump in the primary. Others mulling a bid include former U.S. Rep. Mark Sanford, who is also a former South Carolina governor, and former Ohio Gov. John Kasich.

Weld, on NBC’s “Meet the Press,” was asked by host Chuck Todd if he would step aside if other Republicans ran against Trump.

Weld said he would not get out, “but I’m thrilled about Joe Walsh and Mark Sanford getting in,” though Sanford, still exploring, has yet to launch a formal bid.

“I think that’s terrific. And it’s going to be a more-robust conversation. Who knows? The networks might even cover Republican primary debates. They can ill afford to say they cover only Democrats. But I’m looking forward to both those fellows getting in. And I hope more as well. It can only contribute to more robust dialogue. And that’ll be good for the country. We need to assemble rational people. You know, sure, a crazed president makes the stock market go down. But that doesn’t mean we have to like it,” Weld said.

Walsh kickstarted what now will be a primary campaign with an Aug. 14 New York Times op-ed headlined, “Trump Needs a Primary Challenge,” with the subhead, “The case for a contender from the right.”

In the runup to his bid, Walsh has apologized for a string of provocative and controversial tweets and statements that he’s acknowledged helped pave the way for the Trump era.

In his New York Times piece Walsh wrote, “In Mr. Trump, I see the worst and ugliest iteration of views I expressed for the better part of a decade. To be sure, I’ve had my share of controversy. On more than one occasion, I questioned Mr. [President Barack] Obama’s truthfulness about his religion. At times, I expressed hate for my political opponents. We now see where this can lead. There’s no place in our politics for personal attacks like that, and I regret making them.”