Trump's approach risks eroding global confidence in the U.S., Ackman said.

Bill Ackman, one of President Donald Trump's staunchest defenders among major hedge-fund managers, published a post on X on Sunday urging a "90-day timeout" on some of the tariffs announced by the White House last week.

By slapping both U.S. allies and foes alike with disproportionate tariffs, Trump was risking an "economic nuclear winter" and eroding global confidence in the U.S. as a trading partner, a place to do business and as an attractive destination for investment capital, Ackman said.

Ackman's comments came as U.S. stock futures opened sharply lower on Sunday evening, pointing to a third straight session of stock-market carnage on Monday.

"The president has an opportunity to call a 90-day timeout, negotiate and resolve unfair asymmetric tariff deals, and induce trillions of dollars of new investment in our country," Ackman said.

Otherwise, the U.S. was headed for a "self-induced, economic nuclear winter, and we should start hunkering down."

Trump announced an across-the-board 10% tariff on all goods entering the U.S. on Wednesday. But it was the administration's substantially higher levies applied to many individual nations - both allies and adversaries of the U.S. - that appeared to ignite the stock-market panic. The S&P 500 SPX wrapped up its worst week since March 2020 on Friday.

These higher "reciprocal" tariffs are set to take effect on April 9.

"If, on the other hand, on April 9th we launch 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," he said.

Ackman's comments marked a notable about-face for the billionaire investor. Just days earlier, he had appeared to defend the White House's approach. In a post published on Thursday, Ackman said: "Sometimes the best strategy in a negotiation is convincing the other side that you are crazy."

On Friday, Ackman said he wouldn't be surprised to see Trump on Monday announce a delay in the implementation of the tariffs to give other nations time to make deals.

So far, there has been no indication from the administration that the president would be backing down any time soon.

Over the weekend, Treasury Secretary Scott Bessent dismissed worries that Trump's policies would lead the U.S. into a recession. He said that more than 50 countries had already approached the administration about engaging in talks to strike trade deals.

Trump himself said in a post on Truth Social that he had a productive call with the leader of Vietnam. Reuters reported on Sunday that Taiwan's President Lai Ching-te had offered to roll back tariffs to zero, and that Taiwanese firms would boost investment in the U.S.

Speaking to reporters aboard Air Force One on Sunday evening, Trump said he wasn't intentionally trying to engineer a selloff in financial markets, while insisting that he wouldn't back down.

Pain might be necessary to achieve his goals, Trump said. He said he had spoken with European and Asian leaders, and that they were "dying to make a deal," according to a preliminary transcript from the White House press pool.

"I don't want anything to go down, but sometimes you have to take medicine to fix something," Trump reportedly added.

In a reply to Ackman's original post, NYU economist Nouriel Roubini accused Ackman of acting out of self-interest. He said Ackman had supported Trump's tariff agenda until four days ago, and appeared to be speaking up now only because his personal wealth was at risk.

Ackman addressed Roubini's criticisms in a follow-up post, claiming neither he, nor his firm, Pershing Square Capital Management, had used any margin or leverage in their portfolios that would leave them exposed to a worsening market crash. He added that only a small portion of his portfolio - calls he owns on shares of Nike Inc. - is directly exposed to tariff risk.

More than 90% of Pershing Square's capital is permanent, so the firm doesn't need to worry about redemptions, he said.

Meanwhile, the businesses Pershing invests in are well-capitalized, and to the extent they have debt, it is in laddered maturities.

That said, Pershing doesn't short stocks and doesn't own any market hedges.

"We will suffer mark-to-market losses if the market crashes, but we will not be sellers in a declining market," he said.

A representative for Pershing Square Capital Management declined to comment on Ackman's posts.

Stock futures opened sharply lower on Sunday evening, with S&P 500 futures (ES00) falling more than 5% in the opening minutes of trading. They were down 3.7% in recent trade, while Nasdaq-100 futures (NQ00) were off by 4.7% at 16,717.

Dow Jones Industrial Average (YM00) futures were off by 1,264 points, or 3.3%, at 37,272.

Both the Nasdaq Composite COMP and Russell 2000 RUT fell into a bear market last week, defined as a drop of 20% or more from a recent high. The S&P 500 SPX could enter bear-market territory on Monday if the selling pressure doesn't let up.