Donald Trump is the most protectionist American President since Herbert Hoover, so one of our trade-policy goals of the last three years has been damage control. That’s the best case now for supporting Mr. Trump’s revisions to the North American Free Trade Agreement to gain votes from Democrats.

The good news is that the new U.S.-Mexico-Canada trade deal puts to rest Mr. Trump’s threats to abandon the 1994 agreement and blow up continental trade. The new deal preserves most of the tariff-free trade in the original Nafta. That’s...