BUENOS AIRES—President Trump said he will soon begin the process of terminating the North American Free Trade Agreement to pave the way for congressional consideration of the reworked agreement known as the USMCA.

Termination would start a six-month withdrawal period and give lawmakers a deadline to choose between the new deal or nothing, the president told reporters Saturday aboard Air Force One. His comments came while flying back from Argentina after the Group of 20 summit and a Saturday dinner with Chinese President Xi...