Amidst her presidential bid, Senator Elizabeth Warren proposed a flat tax on the ultra-wealthy: a 2% annual tax on households with a net worth between $50 million and $1 billion, and a 3% annual tax on households with a net worth over $1 billion.

Under this plan, tech moguls like Amazon CEO Jeff Bezos and Facebook CEO Mark Zuckerberg would pay billions in taxes every year in addition to their existing tax payments.

For just one example, in 2019, Bezos would pay over $3 billion on his over $119 billion net worth.

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Some of America's wealthiest tech moguls, including Amazon CEO Jeff Bezos and Facebook CEO Mark Zuckerberg, would face billions in additional taxes under Senator Elizabeth Warren's tax proposal.

The proposed plan is simple: Warren envisions a 2% annual tax on households with a net worth between $50 million and $1 billion, and a 3% annual tax on households with a net worth over $1 billion.

Such a tax would only apply to the ultra-wealthy — a group that includes Facebook CEO Mark Zuckerberg, whose net worth is nearly $70 billion, and Amazon CEO Jeff Bezos, whose net worth is over $110 billion, among other tech moguls.

Warren's campaign calls it the "Ultra-Millionaire Tax."

Facebook CEO Mark Zuckerberg. Getty

According to a new Brookings Institute study, even if Warren's plan had been implemented back in the 1980s, some of the world's richest people would still be quite rich.

Bezos, for instance, would still be worth nearly $90 billion. Zuckerberg would still be worth nearly $45 billion. Google's co-founders, Larry Page and Sergey Brin, would still be worth tens of billions of dollars apiece.

The top 15 wealth holders in the US, from Jeff Bezos to Warren Buffett, would have paid tens of billions in additional taxes.

Read more: Wealth tax explainer: Why Elizabeth Warren and billionaires like George Soros alike are calling for a specialized tax on the ultra-wealthy

The study, co-authored by University of California economics professors Emmanuel Saez and Gabriel Zucman, looks at the theoretical impact of Warren's proposed, "Ultra-Millionaire Tax," which "applies only to households with a net worth of $50 million or more — roughly the wealthiest 75,000 households, or the top 0.1%." It uses the Forbes list of world's richest people as its data source, which began publishing in 1982.

So, what would the list of the ultra-wealthy look like had it been applied back in 1982?

It finds that, everything else unchanged, a wealth tax like Warren's would've brought in over $500 billion of additional tax revenue.

Microsoft co-founder Bill Gates is worth just shy of $100 billion. Thierry Chesnot/Getty Images

Notably, the study doesn't take into account any measures that the ultra-wealthy might've taken to lower their tax burden.

One of the main criticisms of Warren's proposal is that it would be difficult to enforce.

"These are the people who obviously have access to the most sophisticated financial planners and lawyers and accountants," former Department of Justice tax attorney James Mann, who is now a tax partner at law firm Greenspoon Marder, told Business Insider earlier this year. "And I think it's naive to think that they wouldn't plan to minimize their wealth tax burden."