WASHINGTON—Texas Republican Sen. Ted Cruz warned Tuesday that the United States would face “massive economic costs” if President Donald Trump killed the North American Free Trade Agreement — but that Trump might go ahead and make that decision. Cruz said “NAFTA has created millions of jobs across the United States,” and he said “Texans believe in international trade.” He expressed concern that Trump, whom he generally supports, would seek to use the ongoing NAFTA renegotiation to reduce trade and erect protectionist barriers rather than to expand trade. Reducing access to the Canadian and Mexican markets, Cruz said, would “do profound...