After being rebuffed in an attempt to peel back the union protections of federal workers, President Trump took aim elsewhere on Thursday: at their paychecks.

Invoking authority that he and other presidents have used previously, Mr. Trump told Congress he was canceling government pay increases scheduled for next year.

Congress has the power to override his decision, however, and unions representing government workers called on lawmakers to do so.

In a letter to congressional leaders, Mr. Trump said the government would forgo an automatic 2.1 percent pay increase for federal workers scheduled for Jan. 1 and specified that there would be no across-the-board increase for 2019. The letter did not estimate the overall savings from canceling the raises, though it said a related move canceling raises that are based on the workers’ location would save $25 billion.