Maria Bartiromo. Screenshot/Fox Business Donald Trump ripped one of his most frequent critics, billionaire businessman Mark Cuban, in a Friday-morning interview on the Fox Business Network.

Speaking to host Maria Bartiromo, Trump responded to Cuban's claim that if the Republican nominee wins in the fall there was "no doubt in my mind the markets tank."

"Well, I know Mark, and you know the problem with Mark is, he's not smart enough to understand what we're doing," Trump said. "He's really not smart enough."

"I've known him for a long time — he tweets me all the time," Trump said. "He sends me so many different tweets and calls me, although I must say not over the last number of months cause I said this guy ... he'll send out so many tweets. I'll have to send you all of the Mark Cuban tweets and conversations."

Trump doubled down on Cuban being "not smart enough" to understand his economic plans, asking Bartiromo and her fellow Fox Business hosts to "explain to Mark" that "we're in trouble."

"He's a mixed-up guy," Trump said.

In an interview with Business Insider this week, Cuban, the owner of the NBA's Dallas Mavericks and star of ABC's "Shark Tank," said he couldn't think of "anybody more dangerous as president than Donald Trump."

"You've never heard me talk about politics all that much, and it's just — I can't think of anybody more dangerous as president than Donald Trump," he said. "I can't think of anything worse than with him not having a clue. I mean, could you imagine somebody who doesn't read and doesn't learn trying to deal with the day-to-day changes and challenges of that job?"

Cuban endorsed Democratic nominee Hillary Clinton at a rally in Pittsburgh, his hometown, in July. In recent months, Cuban has ripped Trump repeatedly on social media after souring on the Manhattan billionaire's candidacy.