Billionaire hedge fund manager Bill Ackman — a vocal supporter of Donald Trump — on Sunday urged the president to pause his sweeping new tariffs, warning they could economically devastate America if implemented, as planned, on Wednesday.

Ackman, the CEO of Pershing Square Capital Management, claimed on X (formerly Twitter) that while the country is “100% behind” Trump’s mission to fix the global tariff system, he is currently “losing the confidence of business leaders around the globe.”

Trump should announce a 90-day timeout to negotiate new deals, Ackman suggested. Without that pause, he warned, the U.S. will launch an “economic nuclear war” that may cause business investment in America to “grind to a halt,” markets to crash and workers across all sectors to lose their jobs.

“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,” Ackman said. “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,” Ackman concluded. “Alternatively, we are heading for a self-induced economic nuclear winter. May cooler heads prevail.”

Read Ackman’s full post here: