The SDLP - Social Democratic and Labour Party - is the second nationalist party in Northern Ireland and one of the most fervent supporters of the Good Friday Agreement.
Mark Durkan succeeded John Hume as party leader
It draws its support from Catholics and nationalists.
Former party leader John Hume, who stepped down in November 2001 citing ill health, will not seek re-election as Foyle MP, a seat he has held since the constituency was created in 1983.
The party's current leader, Mark Durkan, is expected to contest Foyle where he is expected to face a stiff challenge from Sinn Fein's chairman Mitchel McLaughlin.
One of the party's other two MPs, Newry and Armagh representative Seamus Mallon has also declared he will not seek re-election, leaving the South Down MP Eddie McGrady as the party's only defending candidate.
In the 2003 assembly election, the SDLP lost six seats and was overtaken by Sinn Fein as the main nationalist party.
Many observers believe that if Sinn Fein succeeds in wrestling the Foyle constituency from its grip, it could sound the death knell for the SDLP.
However, there is speculation the recent pressure on Sinn Fein following the Northern Bank raid and the murder of a Belfast man Robert McCartney, both of which have been linked to the IRA, could bring the return of the middle class nationalist vote to the SDLP.
While it supports the principle of a united Ireland, the party stresses that this cannot come about without the consent of the people.
Since an internal review suggested it was losing voters to Sinn Fein because of its "tired and middle class image", it has tried to modernise itself to attract more young and non-Catholic voters.
The SDLP was born in 1970 out of the civil rights movement which came before the Troubles. Many of its founding members, drawn mainly from the Nationalist Party, the National Democratic Party and Republican Labour, were key figures in the civil rights campaigns which grew around Northern Ireland as Catholics demanded an end to what they saw as sectarian government.
The SDLP made history in 1997 when one of its councillors, Alban Maginness, became the first nationalist to be appointed Lord Mayor of Belfast.
The party has historically had close links with Britain's Labour Party.
It takes a strong pro-Europe line and favours entry to the single currency.