Hillary Rodham Clinton yesterday called on her opponent in the New York Senate race, New York City Mayor Rudolph Giuliani (R), to return a $100,000 soft-money donation from a company she called the “number-one polluter in America.” The money was donated by the Renco Group, which owns a Utah company that federal officials have identified as the top toxic chemical dumper in the nation. Clinton campaigned on the theme of the environment throughout the day, meeting with green activists and pledging to fight for more money to clean up the Long Island Sound and investigate a rash of lobster deaths. She also criticized the Giuliani administration for illegal dumping from sewage plants into waterways. For his part, Giuliani claimed that Clinton has ties with executives of Tyson Chicken, a chronic Arkansas polluter.