Social Democratic party (SDP), former British political party founded in 1981 to offer a centrist alternative to the more extreme positions of the then ruling Conservative party on the right and the opposition Labour party on the left. The SDP began with the defection of 12 Labour members of Parliament who opposed their party's leftward drift. The party joined in an electoral alliance with the Liberal party in 1981 and merged with the Liberals in 1988 to form the Social and Liberal Democratic party, now the Liberal Democrats. Some SDP members re-formed the party in 1988, but they disbanded in 1990.
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