WASHINGTON (AP) — The nation’s leading business group on Wednesday raised serious concerns about President Donald Trump’s move to defer Social Security payroll taxes for American workers, warning that the plan for a shot of economic relief during the coronavirus pandemic could prove unworkable.

Chamber of Commerce, a White House ally in battles to cut federal regulations and taxes, said in a letter to Treasury Secretary Steven Mnuchin that Trump’s directive is “surrounded by uncertainty as to its application and implementation” and “only exacerbates the challenges” for companies trying to quickly put his action in place.

The letter also raised questions about whether a tax deferral would have much impact on the economy if workers have to pay the money back.