A right-to-work advocacy group is threatening legal action against the Washington, D.C., transit system if it doesn’t stop deducting union fees from the paychecks of employees without their consent.

The National Right to Work Legal Defense Foundation sent a letter Oct. 30 to the Washington Metropolitan Area Transit Authority alleging that workers are seeing automatic union fee withdrawals from their paychecks without having agreed to them. The deductions violate the U.S. Supreme Court’s June landmark ruling in Janus v. AFSCME...