Second Republican to stand against Trump

Walsh criticised Trump, who has strong support among Republicans, as a bully who is unfit for office as he announced his long-shot bid.

"I'm running because he's unfit," Walsh, 57, told ABC's This Week program on Sunday. "Somebody needs to step up."

"He's a bully and he's a coward and somebody needs to call him out," Walsh said. "The bet ... of my campaign is that there are a lot of Republicans that feel like I do. They're afraid to come forward."

Asked to respond to Walsh's criticism of the president and decision to run, Trump's spokesman replied, "Whatever."

Walsh won a House of Representatives seat from Illinois as a candidate of the Republican Party's fiscally conservative Tea Party movement in 2010, but he was defeated by Democrat Tammy Duckworth in his 2012 re-election bid.

After leaving congress, he became a Chicago-area radio talk show host.

William Weld, a former Massachusetts governor, had been the only Republican trying to unseat Trump but his candidacy has so far failed to gain traction.

Former US Representative and South Carolina Governor Mark Sanford said in July he was considering a primary challenge because of the rising federal debt.

Weld, appearing on NBC's Meet the Press on Sunday, said he welcomed Wash's entry into the race and would welcome Sanford as well.

"It can only contribute to more robust dialogue and that will be good for the country," he said. "We need to assemble rational people. Sure, a crazed president makes the stock market go down but that doesn't mean we have to like it."

Any Republican challenger will face a formidable re-election effort mounted by Trump, who has consolidated his grip on the party's national and state machinery. A Reuters/Ipsos poll conducted on August 19-20 showed 87 per cent of Republicans approve of his performance in office.

Walsh said he was undeterred. "I think this thing ... will catch on like wildfire," he said.

Australian Associated Press