Social Democratic Party
for the German Social Democratic Party see SPD
The Social Democratic Party (SDP) was a British center-left political party of the 1980s. It was formed in 1981 by four senior right-wing Labour Party MPs or ex MPs - Roy Jenkins, Shirley Williams, David Owen and Bill Rodgers, known at the time as the Gang of Four .
The party was formed in reaction to the increasingly-socialist tendencies of the Labour Party, most especially the vote at a Labour conference that massively increased the power of trade unions in party affairs. 
The party eventually attracted 28 Labour MPs and one Conservative Party MP to its ranks. Jenkins and Williams, who were both out of Parliament at the time of the SDP's formation, were re-elected in by-elections.
Later in 1981 the SDP formed an electoral alliance with the Liberal Party. The Alliance fought two elections - 1983 and 1987 - without achieving the breakthrough its members had hoped for. In the General Election of 1983, the party won 26.02% of the electoral vote but won only 23 of 650 seats available.  In 1987 the party's share of the vote dropped to 23.09% and it only won 22 seats. 
To alleviate growing tensions between the party leaders, a merger was agreed in 1988 to form the Liberal Democrats. A handful of Liberals, centered around the Leeds West MP Michael Meadowcroft, declined to merge and kept a small rump Liberal Party in existence, while David Owen and two other MPs maintained a 'continuation' SDP, which, however, was dissolved in 1990.
The SDP sought to 'break the mold' of British two-party politics, but never succeeded in that aim. It achieved, especially in its early years, some spectacular by-election victories but these never translated into progress at general elections. It could be argued that its greatest effect on British politics was to split the center-left vote and enable the Conservative Party under Margaret Thatcher to win the '83 and '87 elections with huge majorities.