WASHINGTON (AP) — President Donald Trump spent more than a year browbeating the leaders of Canada and Mexico into agreeing to a rewrite of North American trade rules. And on Friday, leaders of those two nations are set to sign the pact at the Group of 20 summit in Buenos Aires, Argentina.

Now, Trump faces what could prove a more formidable foe: His own Congress.

Emboldened by their takeover of the House starting next year, many Democrats say they want the new agreement to strengthen its protections for American workers from low-wage Mexican competition. Yet any such changes could raise new obj...