Businessman Mark Cuban continued his crusade against Donald Trump on Tuesday by targeting the presumptive Republican nominee's lack of substance and failed business ventures.

"It’s rare that you see someone get stupider before your eyes, but he’s really working at it," the billionaire and Dallas Mavericks owner told "Extra." "You have to give him credit. It’s a difficult thing to do, but he’s accomplished it.”

Cuban blasted Trump's business ventures, which the presidential hopeful frequently touts as evidence of his acumen and success -- despite the fact that many of them have failed.

“Let’s look at it this way: Name one good deal he’s done," Cuban said. "When he talks about his great renegotiations, they’re renegotiations, so tell me if you think this is a good deal: I lose four casinos, they go out of business, but I’m really good at renegotiating the debt of his companies that have already gone out of business.”

Cuban also unloaded on Trump on Twitter, responding to reports that the candidate's campaign has only $1.3 million cash on hand.

If @realDonaldTrump were fractionally as rich as he says he is,he would write a$200mm check to propel his campaign. He doesn't have the cash — Mark Cuban (@mcuban) June 21, 2016

Trump has continually dismissed any financial woes by boasting that he can self-fund his campaign, but his own disclosures indicate that he does not have enough money to fund a presidential campaign during a general election.

Cuban was once friends with Trump and supported his campaign, even saying that he’d be interested in serving as his running mate.

But he has since soured on him. Cuban has pilloried the real estate mogul in recent months -- for instance, questioning Trump's racist attacks against federal Judge Gonzalo Curiel and his refusal to release his tax returns.

“There’s nothing to preclude you from showing your taxes. That’s your property,” Cuban said earlier this month. “And the fact that you’re being audited — if he’s like me, he gets audited every year. The process is the same, and, you know, it’s really not going to change anything.”

Cuban has also characterized Trump's candidacy as "the campaign about nothing," equating it to an episode of the sitcom "Seinfeld."

“For Trump, it’s The Immigration Episode, The Wall Episode, The Jobs Episode," Cuban said. "His people don’t want to hear the details.”

On Tuesday, he reiterated his concerns over Trump's lack of substantive policy positions.

"At some point, you’ve got to start learning and understanding the issues, you know? Donald has been at this a year but you don’t look at him and say, ‘Wow, he’s gotten so much smarter on this topic or that topic,'" he said. "In fact, you look at him and say, ‘What the hell are you talking about?' That’s not good for America.”

Editor’s note: Donald Trump regularly incites political violence and is a serial liar,rampant xenophobe, racist, misogynist and birther who has repeatedly pledged to ban all Muslims — 1.6 billion members of an entire religion — from entering the U.S.