WASHINGTON (AFP) - As if Hillary Clinton were not formidable enough of a rival, Donald Trump now faces competition from a new quarter - largely unknown Republican dissident, Evan McMullin, who launched his White House bid on Monday.

"It's never too late to do the right thing, and America deserves much better than either Donald Trump or Hillary Clinton can offer us," McMullin, a little-known former CIA counter-terrorism operative, said on his Facebook page.

The 40-year-old McMullin's entrance in the race highlights how the "Never Trump" movement is still actively seeking to upend Trump's candidacy less than three months before the election.

The Republican Party's official presidential nominee has endured a turbulent two weeks in which he suffered many self-inflicted wounds, with increasing numbers of Republicans expressing concerns about Trump's capacity to run the country or voicing outright support for rival Clinton.

"In a year where Americans have lost faith in the candidates of both major parties, it's time for a generation of new leadership to step up," McMullin said.

"I humbly offer myself as a leader who can give millions of disaffected Americans a better choice for president."

McMullin's road will be extremely difficult. The deadlines to get on the ballots in several of the 50 US states has already passed, he has zero national name recognition. Previous "Never Trump" efforts to install a counter-candidate to Trump never got off the ground.

A former House Republican aide, McMullin has been an occasional if fierce Trump critic on social media, calling him an "authoritarian" last month on Twitter.

Conservative group Better For America, which opposes a Trump presidency, is backing McMullin's candidacy, according to ABC News. But as a non-profit "social welfare" group it is not allowed to officially endorse a presidential bid.