President Obama takes his first foreign trip Thursday, but domestic politics will loom large as he tackles the explosive issue of protectionism in a meeting with Canadian Prime Minister Stephen Harper, the leader of the United States’ largest trade partner. At issue is a controversial so-called “Buy American” provision requiring the use of U.S.-produced iron, steel, and other manufactured goods in public works projects funded by the $787 billion economic stimulus bill. Several Democratic-leaning unions and domestic steel and iron producers favor the provision; a large number of business and trade organizations are opposed