Aswath Damodaran, a finance professor at New York University’s Stern School of Business, said the proposal would have created a huge windfall in tax revenue in the first year, when assets are first valued in the market, but after that it would be “a trench warfare year after year to see how much you could raise.”

A billionaire could, for example, move liquid assets like stocks to illiquid assets like real estate and art, where different rules could likely apply (as in the Democrats’ ill-fated proposal).

Others argue that taxing marketable assets, like the shares of executives in their companies (the source of many of the world’s greatest fortunes), gives the government a head start in this game. Such a tax is hard to avoid. because it “directly targets the stocks of publicly traded companies and such data is already being reported,” said Emmanuel Saez, professor of economics at the University of California at Berkeley who studies tax policy and inequality. “There is no way these founders will say they don’t own these companies that they run. “

Theoretically, executives of extremely successful state-owned enterprises could privatize them to avoid such transparency, but it is not an easy undertaking to reduce a personal tax bill. They could also avoid listing companies on the stock market.

Another criticism: taxes with a larger base tend to be more stable. “People will find ways to gamble, and the smaller the tax base, the greater the risk, especially if they are rich, smart and well endowed,” said Shivaram Rajgopal, professor of accounting and auditing at Columbia Business School. “So ideally you want a bigger base if possible. “

For this reason, he prefers the compromise that has survived under the framework proposed by Mr. Biden: an additional 5% tax on annual income above $ 10 million and an additional 3% tax on income above $ 25 million. of dollars. This would apply to about 20,000 people, mostly millionaires, rather than 700 billionaires. “It widens the net a bit,” said Prof Rajgopal.