"He's alienated broad swaths of our population. Donald Trump cannot win," McMullin told ABC News in an interview aired Tuesday on "Good Morning America" when asked about Republicans who say that a vote for him is a vote for Democratic nominee Hillary ClintonHillary Diane Rodham ClintonAmerica departs Afghanistan as China arrives Young, diverse voters fueled Biden victory over Trump McConnell: Taliban could take over Afghanistan by 'the end of the year' MORE.
"He is going to lose to Hillary regardless, he's demonstrated that already," McMullin added when asked if he'd feel guilty playing spoiler to Trump in a few states.
McMullin, 40, who served at the CIA for 11 years before working on Capitol Hill as a policy adviser for the House Republican Conference, launched his White House bid on Monday.
He immediately went after Trump as divisive and personally unstable and slammed Clinton as "a corrupt career politician who has recklessly handled classified information."
"I think they would both be absolutely terrible," McMullin told ABC's Tom Llamas. 
Trump has fallen behind Clinton in a spate of recent polls, trailing the former secretary of State by 7.4 points in the RealClearPolitics average of national polls.