A former CIA counter-terrorism officers is to run for president against Donald Trump.
Mr McMullin, who is the chief policy director of the House Republican Conference, is backed by anti-Trump grup.
The House Republican Conference said in a statement he is no longer employed there.
He said in 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.”
“Authoritarians like @realDonaldTrump use promises of law & order to justify infringing on civil rights as they consolidate control by force,” he posted on 21 July, according to his unverified Twitter account.
The third-party candidacy could also have an unintended affect, rallying anti-establishment Republicans behind Mr Trump.
Mr McMullin is expected to file papers with the Federal Election Commission later on Monday.
