But he reserved particular ire for Trump, calling him "inhuman," and an "authoritarian" who "does not care about anyone but himself."
McMullin, a Republican, previously worked for the House Republican Conference and considers himself a true conservative. But he doesn't see himself as a spoiler for Trump.
"I’m an American first, I'm a conservative next, then I think about party," he said.
"Look, Donald Trump is defeating himself. He is ensuring that Hillary Clinton is elected," he told ABC News today.
"I think they would both be absolutely terrible."
McMullin, 40, did not hold back in his criticism of Trump, saying that one of the latest scandals -- Trump's comments about the speech made by the parents of a fallen Muslim-American soldier -- shows that Trump is "inhuman."
"Donald Trump does not care about anyone but himself," McMullin said. "I do believe he is a fraud and a con man and that’s not something I say lightly."
As for Clinton, he said that her private email server controversy shows "she believes that she's unaccountable to the American people."
McMullin said that he had been hoping that someone with more national name recognition would come forward "many, many months ago" to challenge Trump, but he was approached about his run when "someone came to me," though he did not disclose the name of that individual. He then said that "people" who are deeply upset by Trump's run have discussed his potential candidacy.
"I've been wrestling with the decision for a couple of weeks," noting that he spent "time in prayer" before deciding.
McMullin said that while he just resigned from the House Republican Conference this morning via email, he already has a campaign manager and is optimistic about his campaign finances.
"The money is coming. Americans are responding very quickly," he said.
Though his campaign is not quick enough to play on a completely national level. He has already missed the independent filing deadline in 27 states, but he seems unmoved, saying that they're ready "to fight, to scrape, to claw."
"I’ve spent my life avoiding the limelight and seeking opportunities to serve. Donald Trump has spent his life seeking the spotlight and avoiding opportunities to serve," McMullin said.
"We’re focused on winning. We think the country needs a new generation of leadership."