They invoke the names of Martin Luther King Jr. and Cesar Chavez, but the hundreds of thousands of immigrants who have marched nationwide are not following one charismatic leader. Instead, they're loosely guided by Hispanic advocacy groups, churches and labor unions - organizations that have helped transform what began as isolated campaigns in major cities into a broad movement with a coordinated strategy. "It's a shared leadership among people who we don't even always know," said Nativo Lopez, president of the Mexican-American Political Association, a central organizer of rallies in Southern California. The young movement is still morphing, allowing both...