Arizona Sen. John McCain, 64, has a potentially dangerous form of skin cancer, aides to the politician announced yesterday. McCain has patches of melanoma on his temple and arm, which marks his second bout with the condition. In 1993, McCain underwent surgery for melanoma, which is among the most aggressive and fastest spreading of all cancers. The new lesions were discovered during a routine medical examination earlier this month. The Washington Post reports that the former Vietnam P.O.W., who exited the Presidential race earlier this year and is now active in the George W. Bush Presidential campaign, is now discussing potential treatments with his physicians.