Conservative radio show host and former lawmaker Joe Walsh has become the second man to challenge President Donald Trump for the Republican Party's 2020 White House nomination.
"The country is sick of this guy's tantrum [sic] - he's a child," Mr Walsh, 57, told ABC's This Week, in what is seen as a long-shot challenge.
President Trump made no immediate public comment on the news.
Both men now face an uphill battle to take over a Republican Party that has been refashioned in Mr Trump's image.
No sitting president in the modern era has lost the race to be nominee for their own party.
But both Jimmy Carter in 1980 and George Bush Snr in 1992 faced strong challenges that foreshadowed difficult presidential elections they would lose.
What else did Joe Walsh say?
"I'm running because he's unfit - somebody needs to step up and there needs to be an alternative," he told ABC.
Mr Walsh was elected to the House of Representatives from Illinois in 2010, riding the Tea Party wave.
The Tea Party movement is a conservative group within the Republican Party.
An unlikely 'resistance' hero
It wasn't too long ago that Joe Walsh was a flame-throwing Illinois congressman known best for his invective targeting liberals in general and President Barack Obama in particular. When questioning the Democratic president's birthplace, he sounded a bit like Donald Trump, in fact.
Now Mr Trump is the target of Mr Walsh's sharp words - and he's matching his rhetoric with action. He's said the president is demeaning his office and betraying conservative principles.
The congressman-turned-radio-host makes an unlikely hero for never-Trump Republicans and the left-wing resistance, but they may take their allies where they can find them. And if they view Mr Walsh's politics and inflammatory history as distasteful, perhaps it takes a Trump-style politician to beat Mr Trump.
It would take a cataclysmic shift for Mr Walsh to offer a serious challenge to the president, whose support among Republican voters regularly approaches 90%. For Mr Trump's opponents on the left and the right, however, it may be enough if his new opponent is a television and social media gadfly that gives voice to their anger and gets under the president's skin.
Who will take on Trump in 2020?
Election day is still more than a year away but the race to become the Democratic challenger to Mr Trump is already well under way.
Joe Biden and Bernie Sanders have thrown their hats into the ring, but most of the other candidates are relatively unknown outside the Washington DC bubble.