Former CIA agent Evan McMullin has launched an independent campaign for the White House with support from members of the Never Trump movement.
Mr McMullin, the former chief policy director of the House Republican Conference, is expected to formally file his candidacy on Monday.
He is likely to face challenges on ballot deadlines and fundraising just three months before Election Day.
The 40-year-old Mormon has never held elected office.
Mr McMullin, an outspoken critic of the Republican nominee, Donald Trump, released a statement to ABC News.
"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. I humbly offer myself as a leader who can give millions of disaffected Americans a conservative choice for President," Mr McMullin said.
Who is Evan McMullin?
The little-known, unmarried conservative worked for the CIA for 11 years before leaving the agency in 2010.
He later went to work for Goldman Sachs and became a senior adviser on national security for the House Committee on Foreign Affairs in 2013.
Mr McMullin resigned from his position at the House Republican Conference on Monday, US media reported.
The Utah native graduated from Brigham Young University and earned a master's of business administration from The Wharton School at the University of Pennsylvania, according to his LinkedIn page.
He had just 135 Twitter follows when his pending candidacy was first announced and gained more than 12,000 within hours.
Mr McMullin has voiced his opposition to Mr Trump on social media, calling him an "authoritarian" and denouncing the Republican nominee's attacks on the family of fallen Capt. Humayan Khan.
In a Facebook post, Mr McMullin condemned Mr Trump's anti-Muslim rhetoric, saying "attacking them as a group makes America weaker, not stronger".
Mr McMullin joins third-party candidates Libertarian Party's Gary Johnson and the Green Party's Jill Stein, who are also in the race for the White House.
How could Mr McMullin affect the race?
Mr McMullin faces a series of hurdles over the next three months, having already missed the deadlines to get on the ballot in 26 states.
He will have to collect tens of thousands of signatures in the coming days to make the 10 August deadline for several more states.
But the Morman candidate will have time to appear on the ballot in his native Utah, where Mr Trump is deeply unpopular among the Mormon electorate.
Senator Ted Cruz overwhelmingly defeated Mr Trump in Utah during the primary election, taking 69% of the vote to 14%.
Though Mr McMullin is not likely to appear on majority of state ballots across the country, he could block Mr Trump from taking the historically red state of Utah and peel off some of the billionaire's conservative voters.
Mr McMullin will also have to chase millions of dollars in donations over the next few weeks in order to keep up with his opponents this fall.
Who is behind Mr McMullin?
The group Better for America, which was formed by Never Trump conservatives, had been reportedly searching for an independent presidential candidate to put up against the nominee.
The group has insisted key Republican donors would throw their support behind his candidacy, according to ABC News.
Prominent Republican operative Rick Wilson is expected to help run Mr McMullin's campaign.
Mr Wilson wrote about why Mr Trump needs to "suffer a humiliating defeat" in aNew York Daily News op-ed on Sunday.
He wrote: "if there's a loss by a slim margin in the popular vote or electoral college, millions of already embittered Americans, worked into a frenzy by a shameless leader who will surely refuse to accept the returns, will start the next four years convinced that the United States of America is little more than a banana republic — and the presidency of Hillary Clinton is irretrievably illegitimate".
Florida-based pollster and operative Joel Searby is also thought to be working on the campaign.
Mr McMullin's candidacy comes after the Never Trump movement talked of launching campaigns for Nebraska Senator Ben Sasse and National Review writer David French earlier this year.