Illinois Democrats are hoping to generate about $300 million a year with a last-minute proposal to tax the unrealized gains on investment assets — everything from stocks and bonds to art and collectibles — owned by billionaires in the state.

Two Chicago state representatives leading transit funding talks are floating the new 4.95 percent tax as part of a plan called the Extremely High Wealth Mark-to-Market Tax Act. The money would be used to fund cash-strapped transit systems.

The legislative session is scheduled to end Thursday, so they are trying to rush the legislation through at the last minute.

A think-tank, the Tax Foundation, notes that the tax would also apply to assets held by charities and private foundations. The foundation notes that the Illinois state constitution prohibits taxing personal property, so the new tax is being presented as an income tax.

“If you’re going to go to anybody for funds, why not go to those who are most able to pay and making sure that those folks who have the means will be able to contribute to the system,” a state representative who co-filed the bill, Eva-Dina Delgado, told Capitol News Illinois.

A big issue with taxing unrealized gains — or so-called “paper gains” — is that it is a tax on profits that could vanish before they are cashed in by the owner.

The foundation warns that a person could own a startup tech firm valued at more than a billion dollars that has never turned a profit. The owner of that firm would be considered wealthy enough under the Illinois proposal, but not have the cash to pay the tax without selling a portion of the company.

Another issue would be assigning valuation to items that are not publicly traded like stocks. Privately owned pieces of art, for example, would be difficult to attach a value to for purposes of the tax, the foundation notes.

The sweeping money grab would do more than go after billionaires. The new tax is part of a larger $1.5 billion transit funding plan proposed late Tuesday that would include a 7 percent tax on streaming services, a $5 surcharge on concert tickets, and expanding traffic cameras in the suburbs to raise revenue from driving violations.

The chances of the proposal becoming reality during the current session is small since it ends on Thursday. Even if it passes the lower chamber, it would then need Senate approval before going to Governor JB Pritzker for his signature. Mr. Priztker says he opposes the bill.

“It’s never been done before,” Pritzker told reporters on Wednesday, according to Capitol News Illinois. “Never been done before by any state. Never been done before by the federal government. So once again, you’ve got to do a whole lot of work before you can pass a bill that … you don’t even know how it would work or if you could actually collect on it.”

But, if Illinois does eventually pass a tax on unrealized capital gains, it could drive some of the state’s wealthiest taxpayers to move away and deny the state existing income tax payments, worsening the fiscal picture for the state.

A group of Congressional Democrats have unveiled a similar unrealized gains tax plan at the federal level. They introduced the Billionaires Income Tax Act in September but it has little chance in gaining enough Republican support to pass.