The United States’ top trade official says he’s pushing for changes to continental trade rules to prioritize U.S. content in manufacturing supply chains, but sees a path to preferential tariff rates in North America if Canada and Mexico co-operate with external tariffs on other countries. At the same time, Jamieson Greer warned that negotiations with Canada around the future of the country’s auto sector could be difficult, while discussions about trade in commodities should prove easier. …Canada has not yet started formal talks with the U.S. and won’t be at the negotiating table this week in Mexico City. The three governments have to decide on July 1 whether to extend the agreement for 16 years or move to a period of annual reviews for 10 years. …Ottawa has signalled an openness to this type of “Fortress North America” approach. But Prime Minister Mark Carney wants to see the U.S. lower its sectoral tariffs on automobiles, steel, aluminum, copper and wood products in return for moves toward deeper integration in key sectors. [A Globe and Mail subscription is required for full access to this story]