Several big business groups on Tuesday threatened not to participate in President Trump’s payroll tax deferral plan, calling it “unworkable” unless Congress forgives the tax liability.

“If this were a suspension of the payroll tax so that employees were not forced to pay it back later, implementation would be less challenging … [and not] unworkable,” wrote the group, which includes lobbying heavyweights such as the U.S. Chamber of Commerce and the National Association of Manufacturers, in a letter to congressional leaders and Treasury Secretary Steven Mnuchin.