MEXICO CITY - Mexicans voted Sunday in a tight presidential race to decide whether their country becomes the latest Latin American nation to move to the left, choosing between a shopkeeper's son promising to save the poor and a conservative calling his rival's free-spending populism dangerous. The campaign, which exposed Mexico's painful class divisions, was the first since Vicente Fox's stunning victory six years ago ended 71 years of rule by the Institutional Revolutionary Party, or PRI. Polls predicted a close race between conservative Felipe Calderon, 43, of Fox's National Action Party, and leftist Andres Manuel Lopez Obrador, 52, a...