On 3 March 1988, the Social and Liberal Democrats Party (SLD) was formally constituted, marking the official merger of the historic Liberal Party and the Social Democratic Party (SDP). The move, which followed years of electoral cooperation, created a unified “third force” in British politics, later to be renamed the Liberal Democrats.

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The Liberal-SDP merger was in many ways the logical conclusion of the Alliance, which had contested the 1983 and 1987 general elections on a joint platform.

The Liberal-SDP Alliance had won significant public support, securing 25.4% of the popular vote at the 1983 general election – a mere 2.2 points short of Labour. The vagaries of FPTP translated this to only 23 seats. After a disappointing result in 1987, the Liberal leader David Steel and other senior figures argued that only a full merger could create a credible, unified party capable of breaking the Labour-Conservative duopoly.

The birth of the new party, however, was fraught with difficulty.

The proposal was vehemently opposed by the formidable SDP leader, David Owen. Owen was one of the original “Gang of Four” who left the Labour Party in 1981 in protest at its perceived leftward drift. Owen refused to lead the SDP into the merger, dismissing the new entity as a “ghastly mess, a disaster and a shambles”.

In his resignation statement, delivered after 25,897 SDP members (57.4%) voted for a full merger, Owen said: “We [the SDP] are now deeply and predictably split.”

Some SDP MPs were also sceptical about a merger. A notable exception was Charles Kennedy, who would go on to lead the Liberal Democrats from 1999 to 2006. Meanwhile, Roy Jenkins, Bill Rodgers and Shirley Williams – the remaining members of the Gang of Four – all supported the proposed merger.

Jenkins, who led the SDP from 1982 to 1983, lost his seat at the 1987 general election. Williams and Rodgers both lost their seats at the 1983 general election and failed to return to parliament four years later.

The Liberals enthusiastically embraced the merger, backing a formal coalescence by 998 votes to 21 in September 1987. The party elected a 17-person team to begin negotiations.

The Liberal-SDP merger negotiations proved protracted and acrimonious. A new name for the party was a conspicuous sticking point. The SDP preferred to maintain the “social democratic” label, while the Liberals wanted to keep the word “liberal”.

Michael Meadowcroft, the former Liberal MP and a member of the party’s negotiating team, stormed out of the talks, citing the mooted commitment to Nato in the constitutional preamble for the merged party. Meadowcroft was followed out the door by four of his fellow Liberal negotiators. (Meadowcroft later co-founded the breakaway Liberal Party.)

John Grant, a member of the SDP negotiating team, was another who resigned in frustration.

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An initial policy document, drafted principally by Steel and the new SDP leader Robert Maclennan, was leaked to the press. The widely mocked document, titled Voices and Choices for All, had to be withdrawn on the very day it was due to be published. In an added embarrassment, a specially convened press conference to launch the new party was cancelled with just an hour’s notice.

The document departed from previous Alliance positions; notably, it endorsed the Trident nuclear missile system.

The 1987 Alliance manifesto stated: “We would cancel Trident because of its excessive number of warheads and megatonnage, high cost and continued dependence on US technology.”

The saga damaged Steel’s reputation, reinforcing perceptions that he had a weak grasp of policy detail, and almost derailed the merger altogether. Liberal Party president Des Wilson dismissed the document by declaring it “as dead as John Cleese’s parrot” (after the Monty Python sketch).

A new policy programme, A Democracy of Conscience, relied heavily on the 1987 Alliance manifesto. The document was mocked by at least one Conservative MP as being “cobbled together in about 24 hours”.

The A Democracy of Conscience platform, published on 18 January, was overwhelmingly endorsed by the Liberal Party membership five days later (2099 votes to 385). The SDP membership backed the merger by 273 votes to 28, with 49 abstentions.

The new party was known as the Social and Liberal Democrats (SLD). Steel and Maclennan served as its interim leaders.

Under the interim co-leadership of Steel and Maclennan, the SLD faced both internal divisions and external credibility issues. The party finished a poor third in the July 1988 Kensington by-election on 10.8%, down 6.5 points on the 1987 general election.

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Paddy Ashdown, a former Royal Marine, was elected as the first leader of the Social and Liberal Democrats on 28 July 1988. Neither Steel nor Maclennan stood in the unified party’s first leadership contest.

In his first press conference as leader, Ashdown said his bruised and divided party must “look beyond” internal politics “to the concerns of our nation”. He proposed a “decent, effective and responsible” alternative to Thatcherism.

Ashdown suffered a symbolic setback in the 1989 Richmond by-election.

The continuity SDP, led by David Owen, won 32.2% of the vote, narrowly behind the Conservative candidate, William Hague, on 37.2%. The Social and Liberal Democrats placed third with 22.1% of the vote.

In October 1989, the party was renamed the Liberal Democrats – the unwieldy “Social and Liberal Democrats” had been cruelly reduced to “Salads” by its critics.

In 1990, Owen’s continuity SDP collapsed.

The party finished seventh out of eight candidates in the Bootle by-election, winning just 155 votes. Owen’s outfit had been outperformed by Screaming Lord Sutch’s Official Monster Raving Loony Party.

In the aftermath of the election, Sutch quipped that he would be willing to form an alliance with Owen’s beleaguered party.

The Liberal Democrats under Ashdown won 17.8% of the vote and 20 seats at the 1992 general election.

In the 1992-1997 parliament, Ashdown signalled the end of his party’s “equidistance” between the two main parties, placing the Lib Dems explicitly on the left of British politics and indicating that he would be prepared to work with Labour to defeat the Conservatives.