Robert W. Edgar, who represented Pennsylvania for six terms in the House of Representatives and went on to lead the public interest group Common Cause, died Tuesday. He was 69. Mr. Edgar collapsed Tuesday morning in the basement of his home in Burke after a run on the treadmill, said his wife of 48 years, Merle Edgar. Mr. Edgar, a liberal Democrat, was elected in 1974 in a large class of newcomers that came to Washington after the Watergate scandal. His political career ended after he lost a U.S. Senate campaign in 1986 to Arlen Specter.