Democrats’ plan to tax billionaires excites the party’s base. It could be attractive to Sen. Kyrsten Sinema, D-Ariz. And it might be unconstitutional.

Party leaders have been scrambling to find alternative ways to pay for part of the trillion-dollar-plus “Build Back Better” bill because Sinema has ruled out raising income tax rates on high-end earners and corporations.

A proposal released Wednesday morning would tax the paper investment gains of the ultra-wealthy. The idea is to capture revenue from billionaires whose “tradable” assets — like stocks — appreciate in value each year without being taxed. Under current law, those gains aren’t “realized” and taxed until the underlying assets are sold.

The tax would also apply to the tradable assets of people who earn $100 million or more in three consecutive years, but it would not apply to property such as real estate.

An ally of Biden’s, Rep. Brendan Boyle, D-Pa., a member of the tax-writing Ways and Means Committee, has worked on a version of the wealth tax with Sen. Elizabeth Warren, D-Mass. He said in an interview Tuesday that it is easy for the government to tax all of the earned income of workers, while the country’s wealthiest investors can simply earn value by holding on to untaxed investments.