William Kristol, founder and longtime editor of The Weekly Standard magazine, said Monday he underestimated Donald Trump, but still believes the presumptive GOP presidential nominee is not fit for the White House.
Trump is not qualified because of his "character and temperament," the neoconservative Kristol told CNBC's "Squawk Box."
While criticizing Trump's past business bankruptcies, Kristol said his company is not profitable very often.
Calling Trump a "semi-successful businessman," Kristol said House Speaker Paul Ryan's reluctance to support the billionaire real estate mogul should give voters pause.
Ryan, the 2012 vice presidential candidate, said last week he was not ready to get behind Trump. Trump, in turn, refused to rule out blocking Ryan from serving as the GOP convention's chairman. The two are set to meet this Thursday.
Kristol said he does not want Democrat Hillary Clinton to become president either. "We don't need a binary choice. The system is set up to allow independent candidates to get on the ballot."
Kristol confirmed he held a meeting with Mitt Romney last week to try to draft the 2012 Republican presidential nominee to run this year as an independent or support another third-party candidate.
Romney, while critical of Trump, has said he won't run. In March, Romney delivered a blistering anti-Trump speech, attacking the candidate's temperament, proposals, and adherence to the truth.
For his part, Trump on Sunday noted his support of Romney's 2012 bid for the presidency, saying the former Massachusetts governor is "ungrateful."
On Friday, former GOP presidential candidate Jeb Bush wrote in a Facebook post: "In November, I will not vote for Donald Trump or Hillary Clinton, but I will support principled conservatives at the state and federal levels."
Over the weekend, Trump called Bush out for abandoning a pledge he signed to support the GOP nominee. Trump in September signed the same Republican National Committee pledge.