The U.S.-Mexico trade deal announced by the two countries’ leaders this week now faces scrutiny from their respective legislators, who ultimately must ratify the agreement. The early read: Mexican lawmakers will clear the pact with little trouble, while the U.S. Congress remains wary, especially if Canada doesn’t join soon.

In Mexico, the deal reached during negotiations to revamp the North American Free Trade Agreement will face a vote in a new Senate led by supporters of President-elect Andrés Manuel López Obrador, who has...