ZAMBOANGA CITY—Thousands of supporters, some of them clad in yellow, went wild as showbiz celebrity Kris Aquino took the stage Wednesday night at the Joaquin F. Enriquez sports complex in this famed port city of the South. But Kris, clad in a black top with a small yellow map of the Philippines printed on the side, was not there to promote a show or a movie: she was there to ask the supporters who showed up—numbering at least 50,000—to vote for her brother, presidential aspirant Senator Benigno...