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The SDP-Liberal Alliance was an electoral alliance of the Social Democratic Party (SDP) and the Liberal Party in the United Kingdom that operated from 1981 to 1988, when the bulk of the two parties merged to form the Social and Liberal Democrats, later referred to as simply the Liberal Democrats.

Following the establishment of the SDP by dissidents from the Labour Party in March 1981, the new party entered into an informal alliance with the existing centre party, the Liberals, led by David Steel. The SDP fought its first by-election, in Warrington, with future leader Roy Jenkins standing as "SDP with Liberal support".

By the autumn of 1981, this arrangement was formalised into an alliance, with both parties agreeing to stand down in each other's favour. Between 1981 and 1983, the parties together won seats in by-elections in:

• Croydon North West (Bill Pitt, a Liberal),
• Crosby (Shirley Williams of the SDP),
• Glasgow Hillhead (Roy Jenkins of the SDP), and
• Bermondsey (Simon Hughes of the Liberals, with the largest swing ever recorded in any British election).

The Alliance won 25.4% of the national vote in the 1983 general election, compared to Labour's 27.6%. Only 23 Alliance Members of Parliament (MPs) were elected, compared to Labour's 203. The SDP came second in many constituencies, but Britain's first past the post electoral system meant that this success did not translate into parliamentary seats. For many years after, both the SDP and the Liberal party were committed to proportional representation, under which system they would have had a much larger presence in the House of Commons.

However, the alliance failed to maintain momentum. In the 1987 general election, the share of the vote for each party fell slightly; the Liberals won 17 seats (the same as in 1983), while the SDP won five (one fewer than four years previously).

By 1987, relations had become fraught. David Steel proposed a merger of the two parties in following the 1987 general election. The majority of both parties agreed, and a merger was effected early in 1988.

Small factions of both parties established new parties under the names of the SDP and the Liberal Party but, as each was made up of those members who least trusted the other party, there was no chance that the two 'continuing' parties would co-operate.

The SDP was re-established under the leadership of one of the founders of the SDP, David Owen. (See : Social Democratic Party (UK, 1988).) Owen and the party's national executive dissolved the party in 1990, but another group of party members continue the party under the SDP name. (See : Social Democratic Party (UK, 1990).)

The Liberal Party was re-established under the leadership of Michael Meadowcroft, and continues to operate. (See: Liberal Party (UK, 1989).)