MEXICO CITY — President Vicente Fox and popular Mexico City Mayor Andres Manuel Lopez Obrador exchanged barbs during separate speeches Saturday in an ongoing political battle whose outcome could have a major impact on the 2006 presidential election. Fox, in his weekly national radio program "Fox With You," repeated his now-routine warnings against a return to "populism" and the state-run programs of Mexico's past, an indirect, but obvious reference to the mayor, whose popularity stems in large part from his large-scale handout programs and public works projects. The mayor also supports a return to a more oil-based, state-supported economy. Exercising...