WASHINGTON • Elon Musk isn’t happy.

With a personal fortune that is flirting with $300 billion, the Tesla CEO — the richest person on earth — has been attacking a Democratic proposal to tax the assets of billionaires like him.

The idea behind the Democratic plan is to use revenue from a billionaires tax to help pay for a domestic policy package being negotiated in Congress that would, among other things, help combat climate change, provide universal prekindergarten and expand health care programs.

Musk, who recently blew past Amazon founder Jeff Bezos as the world No. 1 in wealth thanks to Tesla’s soaring share price, would be liable for perhaps a $50 billion tax hit under the Democratic proposal.

Forget it, he says.

“My plan,” the SpaceX founder tweeted Thursday about his fortune, “is to use the money to get humanity to Mars and preserve the light of consciousness.”

He might well get his wish. Prospects for the billionaires tax appear to be dimming fast in Congress. The pivotal Sen. Joe Manchin of West Virginia is signaling his opposition to the plan along with some others, including some of his fellow Democrats, who have described such a tax as logistically impractical.

This week, Musk argued, the fundamental problem is that government spends too much money — and he warned that the billionaire tax proposal could lead over time to tax hikes for more Americans.

“Eventually,” he tweeted Monday, “they run out of other people’s money, and then they come for you.”

The Democratic proposal, unveiled Wednesday by Sen. Ron Wyden, would tax the gains of people with either $1 billion or more in assets or three consecutive years of income of $100 million or more. It would apply to fewer than an estimated 800 people, who would have to pay tax on the value of tradable items, like stocks, even if they don’t sell them.

Under current law, such assets are subject to tax only when they’re sold.

Supporters have said the tax could raise $200 billion over 10 years that could help fund Biden’s legislative priorities. Republicans are unified in opposition to the proposal.

And some have suggested it would be challenged in court.

The Democrats’ proposal comes against the backdrop of growing concerns about vast economic inequality, with the wealth of many American multibillionaires having accelerated during the COVID-19 pandemic, thanks to increased stock and home equity, even more than before the virus struck.

John Catsimatidis, the billionaire grocery chain and real estate magnate who owns Gristedes, condemned the proposal as something you would “expect Putin to do,” referring to President Vladimir Putin of Russia.

The billionaire tax plan, Catsimatidis said, is “a little bit insane.”

“The American people have reached the point where they’re saying, ‘Enough is enough’,” said Catsimatidis, who lost a bid for the Republican nomination for New York City mayor in 2013.

Tesla CEO Elon Musk said President Joe Biden’s tax plan proposal on billionaires is insane.

The Associated press file