• Billionaire investor Mark Cuban insisted that Vice President Kamala Harris would not tax unrealized capital gains as president.
• Harris has already broken with President Joe Biden on tax policy by proposing a smaller hike in the top capital gains rate.
• Harris is teasing out her policy plans in the days leading up to her first, and possibly only, debate with Republican nominee Donald Trump.

Billionaire investor Mark Cuban on Thursday insisted that Democratic presidential nominee Kamala Harris would not tax unrealized gains as president.

"Every conversation I've had is that it's not going to happen," Cuban said on CNBC's "Squawk Box."

Cuban, who says he speaks with Harris' team frequently, maintained to CNBC that she is not interested in taxing unrealized gains.

He cautioned, "I'm not going to speak for the vice president, she makes the final decision."

Still, "I'm talking to these folks three, four times a week, having back-and-forth conversations, and their verbatim words to me is, 'That's not where we want to go.'"

"We need to find alternative sources of revenue," Cuban said Harris' aides have told him, "and those alternative sources of revenue are meant to replace what the unearned income -- the unrealized gains tax from the Biden plan would have implemented."

Cuban is neither a Republican nor a Democrat, but he was one of more than 100 venture capitalists earlier this summer who endorsed Harris for president.

Cuban's remarks could signal another break on tax policy between Harris and President Joe Biden, who dropped out of the presidential race in July and endorsed the vice president as his successor.

Biden's fiscal 2025 budget plan proposes a 25% minimum income tax on Americans with wealth above $100 million.

Unlike current law, Biden's budget would apply an annual tax on unrealized gains — the increased values of assets that have not been sold — for the richest Americans. The plan has received pushback from Republicans and even some Democrats.

Harris, who took over the Democratic ticket less than four months before Election Day, has not explicitly ditched the plan to tax unrealized gains.

In a speech Wednesday rolling out more of her economic policies, however, Harris said she supports a "billionaire minimum tax."

Harris, said Cuban, is "starting from the Biden plan as a starting point, but that's not necessarily her ending point."

The Harris campaign did not immediately respond to CNBC's request for comment on Cuban's remarks.

Harris, whose nascent campaign has sought to appeal to moderate voters, on Wednesday veered away from the Biden budget's plan to raise the capital gains tax to 39.6% from 23.8% for households making more than $1 million.

Harris' plan would instead increase the top long-term capital gains rate to 28%. The Harris campaign in a press release Wednesday noted that that figure is "well below the rate proposed" in the Biden budget.

"In her view, this approach strikes the right balance," the campaign said.

Harris is teasing out her policy plans in the days leading up to her first, and possibly only, debate with Republican nominee Donald Trump, which is set for next Tuesday.

Disclosure: CNBC owns the exclusive off-network cable rights to "Shark Tank," which features Mark Cuban as a panelist.