Billionaire hedge fund investor Bill Ackman, who endorsed President Trump last year, is offering a stark warning about the risks of the White House’s tariff plans, arguing they could cause the economy to collapse while hurting Trump supporters the most.

“Business is a confidence game. The president is losing the confidence of business leaders around the globe. The consequences for our country and the millions of our citizens who have supported the president — in particular low-income consumers who are already under a huge amount of economic stress — are going to be severely negative,” he wrote.

Ackman began a lengthy post on the social platform X by praising Trump’s strategy, arguing the nation is “100%” behind Trump on fixing a trading system that disadvantages the country.

But while Trump has elevated the issue and gotten everyone’s attention, “by placing massive and disproportionate tariffs on our friends and our enemies alike and thereby launching a global economic war against the whole world at once, we are in the process of destroying confidence in our country as a trading partner, as a place to do business, and as a market to invest capital,” Ackman wrote.

He argued that Trump should consider calling a “90-day time out” that would allow him to negotiate and solve “asymmetric tariff deals, and induce trillions of dollars of new investment in our country.”

If that doesn’t happen and instead the U.S. launches “economic nuclear war on every country in the world, business investment will grind to a halt, consumers will close their wallets and pocket books, and we will severely damage our reputation with the rest of the world that will take years and potentially decades to rehabilitate,” Ackman wrote.

“What CEO and what board of directors will be comfortable making large, long-term, economic commitments in our country in the middle of an economic nuclear war? I don’t know of one who will do so.”

Ackman predicted that if markets crash, “new investment stops, consumers stop spending money, and businesses have no choice but to curtail investment and fire workers.”

So far, markets have plunged for two days in reaction to the large tariffs Trump announced, with futures down significantly on Sunday evening ahead of the Monday morning open.

Ackman argued big companies as well as small and medium-sized businesses and entrepreneurs will suffer pain if the negative consequences unfold. And he predicted prices will rise, something Trump officials on Sunday were downplaying.

“Almost no business can pass through an overnight massive increase in costs to their customers. And that’s true even if they have no debt, and, unfortunately, there is a massive amount of leverage in the system,” he wrote.

Ackman also warned that Trump risks causing negative consequences for those who support him.

“This is not what we voted for. The President has an opportunity on Monday to call a time out and have the time to execute on fixing an unfair tariff system. Alternatively, we are heading for a self-induced, economic nuclear winter, and we should start hunkering down. May cooler heads prevail.”