Mark Cuban warns taxing unrealized gains will 'kill the stock market,' insists Harris won't actually do it

Billionaire Mark Cuban acknowledged that Vice President Kamala Harris' plan to tax unrealized capital gains would "kill the stock market," while insisting that she won't go through with it as dubious CNBC hosts pushed back in a contentious interview Thursday.

Cuban joined CNBC's "Squawk Box" on the phone to discuss his support for Harris, revealing that he regularly communicates with her campaign to advise on economic issues.

The interview took a turn when Cuban was asked about the Biden administration’s proposal – which the Harris campaign has indicated it supports – to tax investment returns that have not yet been realized.

Cuban said he disagreed with the premise of taxing unrealized gains, but insisted that it would not happen under a Harris presidency. He said the Harris campaign is using the Biden tax plan as a "starting point," but that it's "not necessarily her ending point."

HARRIS BREAKS WITH BIDEN ON CAPITAL GAINS TAX; CALLS FOR TOP RATE OF 28%

"What I told them is if you tax unrealized gains, you’re going to kill the stock market, and it’s going to be the ultimate employment plan for private equity because companies are not going to go public because you can get whipsawed," the "Shark Tank" investor said.

He added that in the early stages of a company's development, it is common to be "cash poor" despite having equity.

THE KAMALA HARRIS TAX PLAN

"My own personal experience back from the internet days, all of a sudden… my net worth was enormous but the number of dollars in the bank wasn't enormous," he said, "Based off the unrealized gains, I would have had to borrow money and I effectively would have been in hock just to pay my tax bill instead of trying to run my company."

"They realize that's the issue," Cuban claimed. "I can't repeat it enough, even though she's not directly conflicting, the Biden tax plan, to her, her value proposition is we need to tax everybody fairly."

Reports from The Wall Street Journal and The New York Times have said that Harris supports the tax increases put forth by President Biden in his recent budget proposal. This, according to Harris’ campaign, includes a 25% tax on unrealized capital gains for individuals with more than $100 million in wealth, and an increased corporate tax rate from 21% to 28%. The move to tax unrealized gains is in line with the Biden-Harris administration's promise to raise taxes on the wealthy and corporations.

The Harris-Walz campaign reportedly told Marc Goldwein, vice president of the Committee for a Responsible Budget, that it supports the tax increases on high earners proposed by Biden.

CNBC host Rebecca Quick questioned why the Democratic nominee has not yet publicly quashed concerns and explained herself. She also suggested that the campaign might be telling high profile donors, such as Cuban, what they want to hear, regardless of the position they plan to take.

"As you've said yourself, you can't speak for the vice president. These are things that they're telling you. Who knows what they're telling other people?" Quick asked Cuban. "My guess is that they're telling anybody who is donating to them exactly what they want to hear at this point. What matters is what they say publicly and what they will stick to."

"No, no, no, no, no, no. Absolutely, positively not," Cuban interjected.

"Why do you think they’re telling you the truth and not telling other people other things? If they won’t say it publicly. Say it publicly so we can hold you to it," Quick pressed.

She said voters shouldn't have to "trade on what we think and what we feel."

"Like any good CEO, you’ve got to do it when you get it right. Not because your customer is yelling, ‘Well, we’re waiting on new pricing,'" Cuban replied.

"Yeah, but we have 60 days to an election," Quick said.

Cuban went on to argue that Harris has failed to comment on the issue because it only impacts "less than 1% of the voting population."

"This is not [a] number one priority for her, it’s not, in terms of speaking about it and getting the word out. She wanted to talk about entrepreneurs, startups, founders first," he said. "The people like me in my position, we’re not at the top of the list. We’re closer to the bottom, so we’re not going to get that full, detailed disclosure first, and I’m okay with that."

Earlier this week, Quick and her co-host Joe Kernen clashed with Bharat Ramamurti, an informal economic adviser on Harris' campaign, over the Biden-Harris proposal to tax unrealized gains calling it "unconstitutional."

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Overall, the unrealized gains tax, along with the substantial capital gains tax increase, are projected to lead to nearly $800 billion in new government revenue, according to a Peter G. Peterson Foundation analysis. 

The Harris campaign did not respond to Fox News Digital's request for comment.

Fox News' Hannah Penreck and Alec Schemmel contributed to this report.