Mark Cuban said that taxing unrealized capital gains would "kill the stock market."
President Joe Biden proposed taxing unrealized gains for people worth over $100 million.
Kamala Harris is unlikely to endorse Biden's plan, Cuban said.
Any proposal to tax unrealized capital gains would "kill the stock market," the billionaire investor Mark Cuban said in a CNBC interview on Thursday.
As part of his wide-ranging tax proposals, President Joe Biden has suggested taxing unrealized capital gains for people with a net worth of more than $100 million.
While Vice President Kamala Harris has not endorsed or dismissed Biden's proposal on unrealized capital gains, Cuban said, it's dead on arrival.
"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," Cuban said.
Cuban's "whipsaw" comment alluded to the main question investors have surrounding proposed taxes on unrealized capital gains: What happens if those unrealized capital gains eventually turn into unrealized capital losses in a volatile stock market?
But according to Cuban, who said he'd been talking with the Harris campaign often in recent weeks, Harris is highly unlikely to endorse such a plan.
"They realize that's the issue," he said, adding of Harris: "Even though she is not directly conflicting the Biden tax plan, to her, her value proposition is we need to tax everybody fairly, starting from the Biden plan as a starting point. But that's not necessarily her ending point."
Harris has already rejected some aspects of Biden's tax proposals, offering her own vision of what she would propose as president.
While Biden proposed to move the long-term capital-gains tax rate to 39.6% for households with taxable income of more than $1 million, Harris says that's too high and has proposed raising it to 28% instead.
"The point I'm really trying to convey is: She's open-minded. She's not an ideologue. She wants to do what's best for business," Cuban said.
Cuban defended the Democratic presidential nominee despite criticism that Harris has yet to unveil a slew of detailed economic-policy proposals with the November election fast approaching.
"Like any good CEO trying to turn around a battleship, there's only so much you can do every single day," Cuban said. "Like any good CEO, you've got to do it when you get it right."
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