He was co-founder of the Irish Anti-Partition League (APL) in 1946 and staged a nationalist councillors’ protest during a visit by the governor of Northern Ireland in 1951. He became leader of the nationalist parliamentary group of ten members in 1964. They became the official opposition at Stormont in February 1965.
He predicted that the decision to ban a civil rights march in Derry in 1968 would “prove the greatest mistake by a Stormont government”. He was defeated by John Hume in the 1969 election and never held public office again.