A theory gaining ground in Washington is that President Trump has begun to grasp the economic importance of the North American Free Trade Agreement (Nafta) to his Trump state constituents and political future. You wouldn’t know it by watching U.S. Trade Representative Robert Lighthizer negotiate the final stages of the Nafta rewrite with Mexico and Canada.
Legislatures in all three countries will have to approve the amended pact and that won’t be easy this year with midterm elections in the U.S. and presidential and congressional...