Republican presidential candidate Marco Rubio says Donald Trump is just not yet ready to serve as Commander in Chief.

Last week, former Florida Governor Jeb Bush called Trump’s views on national security “dangerous” and doubted his readiness to serve as president. In an Op-Ed penned for the National Review, Bush wrote, “Donald Trump simply doesn’t know what he’s talking about. And his bluster overcompensates for a shocking lack of knowledge on the complex national-security challenges that will confront the next president of the United States.” He added in an interview with CNN, “Across the spectrum of foreign policy, Mr. Trump talks about things as though he’s still on ‘The Apprentice.'”

In an interview with CNN on Monday, Rubio refused to agree with his Republican rival. He did, however, say that “up to this point” Trump has yet to clearly outline a “deep understanding of the issues before this country in a serious way.”

“Ultimately the next President of the United States, on their first day in office, must understand the threats that face this country and must have shown good judgment about what to do about those issues,” he stressed. “To this point in the campaign, he has not proven an understanding of these issues or the preparation necessary to be the Commander in Chief of the most powerful military force in the world.

But Rubio added, “Obviously he has time to change that.”