The law passed by Congress -- the North American Free Trade Agreement Implementation Act -- is critical because it directs the president to implement the terms of the NAFTA agreement.

"When it consented to NAFTA, Congress passed a statute called the NAFTA Implementation Act, which delegates to the president the authority and responsibility to implement NAFTA," Vanderbilt University law professor Timothy Meyer told The Daily Caller.

Only Congress can repeal the statute implementing NAFTA.

Paradoxically, then, Trump would still be bound to continue implementing the NAFTA law Congress passed even if he decides to formally withdraw the United States from NAFTA as an international agreement.