* John P. Roche, a political scientist, counselor to Presidents John F. Kennedy and Lyndon B. Johnson, and professor emeritus in American civilization and foreign affairs at Tufts University, died Friday at Youville Hospital in Cambridge, Mass., of complications from a stroke. He was 70. A prolific author of books and articles and a syndicated columnist, Mr. Roche was active in the civil-rights movement in the 1950s. He was a co-founder of Americans for Democratic Action and served as its president from 1962 to 1965. In the 1960s, he also wrote speeches for Vice President Hubert H. Humphrey.