SDLP
On 21 August 1970 Cooper co-founded the Social Democratic and Labour Party (SDLP) with Hume, Paddy Devlin and Gerry Fitt.
Cooper organised a civil rights and anti-internment march for 30 January 1972 which was to develop into Bloody Sunday, whereupon fourteen unarmed civilians were killed by soldiers from the Parachute Regiment on duty in Derry.
After the prorogation of the Stormont Parliament, Cooper was elected as one of the representatives of Mid Ulster to the Northern Ireland Assembly, 1973 and the Northern Ireland Constitutional Convention in 1975. He was also the SDLP's candidate in the constituency in both the February 1974 and October 1974 Westminster elections. By standing in the first of these, he split the nationalist vote and in effect ensured the defeat of independent MP Bernadette McAliskey.
In 1983, Cooper stood aside after the boundary changes for the new Foyle constituency to let colleague and friend John Hume contest the seat. The increase in levels of violence intertwined with the politics made Cooper slowly move away from politics. He is now an insolvency consultant.
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