Evan McMullin, who reportedly has the backing of some key Republican donors, has been a member of the “Never Trump” movement within the party. He will stand as an Independent.
He said: “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.
“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.
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“As Donald Trump continues attacking Muslims and as a former CIA officer, I’d like all Americans to know the truth: American and other Muslims have played a central role in virtually every counter-terrorism win we’ve had since 9/11. They are an indispensable asset in this fight. Attacking them as a group makes America weaker, not stronger.”
Mr McMullin has been a frequent critic of Mr Trump on social media, calling him an authoritarian and criticising his stance on civil rights as well as his refusal to release his tax returns.
A May 7 post to an unverified Twitter account in his name said: “Opposing Donald Trump is about putting principle over power, a virtue some in Washington are too quick to abandon.”
Like Mr Trump, however, Mr McMullin has never held elected office.
He most recently served as the House Republican Conference’s chief policy director and has worked in Congress since 2013.
“The House Republican Conference has zero knowledge of his intentions,” conference spokesman Nate Hodson said.
He previously spent 11 years as an operations officer for the Central Intelligence Agency.
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Mr McMullin attended the same business school as Mr Trump, although not at the same time.
Mr McMullin’s bid was first reported by Joe Scarborough, a former Republican congressman and high profile US tallk show host, who has been in an on-going feud with Mr Trump.
He said the McMullin campaign would have “more to do with stopping Donald Trump than actually electing a president”.
Deadlines for an independent candidate to get on the presidential ballot in each state are fast approaching.
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The states have different deadlines but Mr Scarborough said Mr McMullin would aim to get on the ballot in between 20 and 30 states.
In order to get into the three upcoming presidential debates he would have to achieve 15 per cent in five national polls.
Mr McMullin was an unknown quantity across America but his announcement quickly led to a surge in his Twitter followers from 117 to over 10,000.
It remained to be seen who his backers within the Republican Party were, or how much of a focus he would become for anti-Trump Republican sentiment.
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Any Republicans voting for him would do so in the knowledge they would be helping Democratic nominee Hillary Clinton.
When he was in the CIA Mr McMullin managed clandestine counter-terrorism operations. He later worked at Goldman Sachs.
His move represented the latest evidence of distrust of Mr Trump in the intelligence community. Last week Mike Morell, former acting head of the CIA, accused Mr Trump of being an “unwitting agent” of Russia.