Jeb Bush did not know that his campaign was doing its due diligence to possibly break from the GOP pledge to support the party’s eventual nominee in the wake of Donald Trump’s repeated threats to run as a third-party candidate, the GOP candidate said in an interview late Thursday.

But he was able to explain it.

Appearing on CNN’s “Anderson Cooper 360,” the former Florida governor was asked by anchor John Berman why his campaign would consider doing such a thing.

“Well, because Donald Trump had threatened to go, once again, to become a third-party candidate. And so, I didn’t know they were doing this, but that’s the smart thing to do in the campaign is to determine exactly what the consequences are for making that kind of decision,” Bush said. During Tuesday night’s debate, Trump reiterated that he would not seek a third-party bid, citing his strong performance in various state and national polls.

As the calendar inches closer to the Iowa caucus and New Hampshire primary in early February, Bush said, “Trump will begin to fall because he’s not a serious candidate.” 

“Listen to his comments as it related to the nuclear triad, for crying out loud. This is the pinnacle of our, of our deterrent to be able to keep, be kept safe, since the post-World War II era. He had no clue when Hugh Hewitt asked him that question,” Bush remarked, in reference to the Manhattan businessman’s struggle to explain the challenges of an aging nuclear defense and delivery system. “If he is going to be president of the United States, he needs to be a serious person, not someone who views this as some kind of entertainment show.”

The main point of such tactics, Bush suggested, is that Trump will not be the nominee because “he's not a serious candidate.” Bush has taken to calling Trump the “chaos candidate,” painting the mogul as someone who could not be trusted with making serious decisions. (Bush used the word “serious” at least a dozen times during the Tuesday debate.)

On Thursday, the super PAC supporting Bush unveiled a new ad hailing the former Florida governor’s debate performance against Trump. The ad, paid for by Right to Rise USA, starts out with a voiceover that says, “One candidate tough enough to take on the bully.”

Bush echoed that line on CNN.

“And all of the other candidates seem to be intimidated by him, but I’m not. He’s a bully. He’s not a serious candidate,” he told Berman. “He has not offered compelling alternatives to the path that we’re on, and we desperately need to change direction as it relates to our national security.”