Frank Mankiewicz, a writer and Democratic political strategist who was Senator Robert F. Kennedy’s press secretary, directed Senator George S. McGovern’s losing 1972 presidential campaign and for six years was the president of National Public Radio, died Thursday at a hospital in Washington. He was 90. The cause was heart failure, said Adam Clymer, a former New York Times reporter who is the spokesman for the family. Mr. Mankiewicz had been in the hospital’s intensive care unit for more than three weeks receiving treatment for heart and lung problems, Mr. Clymer said. A scion of Hollywood, the son of Herman...