– Former Rep. Joe Walsh said Sunday he would mount a GOP presidential primary challenge because, "We can't take four more years of Donald Trump."

On ABC's "This Week," the former Illinois congressman, now a radio talk-show host, called Trump "completely unfit to be president." He said he was launching his quixotic bid because "nobody in the Republican Party stepped up" despite what he described as deep dismay over Trump's performance. "He's nuts. He's erratic. He's cruel. He stokes bigotry. He's incompetent. He doesn't know what he's doing," Walsh said.

"Everybody believes — in the Republican Party, everybody believes he's unfit," said Walsh, a Tea Party stalwart with his own history of inflammatory comments who formerly represented a suburban Chicago district before losing his seat in 2012. "He lies every time he opens his mouth."

Walsh's announcement that he would contest the 2020 GOP nomination followed a head-spinning week, even by the turbulent standards of Trump's administration. Over a period of days, the president engaged in a series of striking policy gyrations and a stream of provocative commentary.

Through it all, leading congressional Republicans were almost uniformly silent. Walsh said that left him ­convinced that the party was out of step with voter perceptions, despite polling that indicates strong support for Trump among Republican voters.

The party's national committee has thrown its machinery into high gear in support of the president, and Trump backers are in firm control of GOP party leadership in key primary states.

A few other Republicans have talked about challenging Trump, including former Rep. Mark Sanford of South Carolina, Maryland Gov. Larry Hogan and former Sen. Jeff Flake of Arizona. Former Ohio Gov. John Kasich, who ran against Trump in the 2016 primaries, has also talked about a rematch but appears to have decided against it.

Walsh has a history of rhetoric that has sometimes targeted ethnic, religious or racial minorities in a manner reminiscent of Trump's. In December 2016, he tweeted "Obama is a Muslim." Asked if he truly had believed that, he responded, "God, no."

"I helped create Trump," Walsh told George Stephanopoulos. "And George, that's not an easy thing to say."