The SDLP MP, Mr Eddie McGrady, is not seeking renomination to contest the Assembly election scheduled for May 1st.
His intention to stand down from Stormont is the latest retirement of many founding members of the party from the Assembly. It follows the resignation of Mr John Hume and declarations from Mr Seamus Mallon and the party deputy leader, Mrs Brid Rodgers, that they will not seek re-election.
Mr McGrady told The Irish Times yesterday he had "no intention at this stage" of reconsidering his Westminster career or quitting politics.
Seen by many in the party as a pillar of the organisation since its inception, he was the first SDLP chairman and has contested all Northern elections since 1969.
Mr McGrady has, at various times, been elected to his local council in Co Down and to the various Stormont assemblies since 1974. He served as a minister in the first power-sharing assembly established under the Sunningdale agreement in 1974. He was also elected to the constitutional convention in 1975, the "rolling devolution" assembly set up by the then Northern Secretary, Mr James Prior, in 1982 and the Forum in 1996.
He won the South Down Westminster seat from the late Mr Enoch Powell in 1987 at his fourth attempt. He took the seat from the UUP candidate with a majority of just over 800 votes. He has since made it a safe nationalist seat.
Mr McGrady insisted last night he was "not leaving the scene". Rather it was a case of "rationalising the amount of work I have to do". He is a member of various Westminster committees includ- ing the all-party committee on Northern Ireland.
The SDLP in his constituency will select candidates to defend the three seats it holds on Sunday.
Sinn Féin leaders yesterday began a four-week tour of the Republic to urge people to push strongly for the full delivery of the Belfast Agreement.
Senior leadership figures have been lined up to address public meetings over the coming weeks. The party's Assembly group leader, Mr Conor Murphy, was due to kick off the series of events in Enniscorthy, Co Wexford, last night .
The party chairman, Mr Mitchel McLaughlin, is also due to make keynote speeches in Limerick today and Cork tomorrow.
There will also be events in Cos Monaghan, Galway, Kilkenny and Waterford, culminating in a meeting in Dublin next month which will be addressed by Mr Martin McGuinness.