The Social Democrats' short-term impact was remarkable. Contemporaries likened it to a giant rocket lifting off from Cape Canaveral or the volcanic eruption that had blown apart Mount St Helens in the American West only a few months before. One member of the party's founding Gang of Four, Roy Jenkins, nearly captured the safe Labour seat of Warrington in a 1981 by-election. Shirley Williams, captured Crosby from the Conservatives in November of that year. In December 1981, nine months after the SDP's launch, a Gallup poll showed the party, in alliance with the Liberals, well ahead of Labour and the Tories.
The SDP was also astonishingly successful as an essay in political mobilisation. As a new party largely devoid of dogma and unfettered by established interest groups, it attracted into politics some some 60,000 "political virgins", whose enthusiasm often extended beyond writing cheques to take in intense local activism.
SDP support ebbed in 1982 following the Falklands war and Britain's partial recovery from the first Thatcher recession. Despite this, the SDP-Liberal Alliance, led by Roy Jenkins and David Steel, captured 25.4 per cent of the popular vote at the 1983 general election - the highest third-party vote for 60 years - and almost overtook Labour. As late as 1987, only months before the SDP's demise, the Alliance, by now headed by the two Davids, Owen and Steel, captured as much as 23.2 per cent of the vote.
But it was obvious by then that the party was never going to "break the mould" of British politics, as Roy Jenkins had hoped. The Alliance won only 23 seats in 1983 and 22 in the next general election four years later. It never held the balance of power. It certainly failed to displace Labour as the main opposition to the Tories. A Rip van Winkle who awoke today, having slept for the past 20 years, would find that the British party system had scarcely changed since he nodded off. In November 1975, the Liberals' Gallup poll standing was 12.5 per cent. Today, the Liberal Democrats' is 14.5 per cent. The SDP has disappeared virtually without trace.
What went wrong? One theory was that the SDP lacked vision. As one Social Democrat MP liked to say of the leadership at the time, "They don't what tune we're whistling." Another theory was that the party was destroyed by internal wrangling, as the nice party turned nasty. Certainly the SDP's end, in 1987-88 came amid scenes of tragi-comical farce. Shirley, Roy and Bill Rodgers wanted to merge with the Liberals; David Owen didn't. They threw tantrums, he sulked, and then, to everyone's astonishment, went off to form his "continuing SDP", a rump party dedicated to his leadership. By the time that collapsed, the merger had been all but killed off by David Steel and Robert Maclennan's "dead parrot" - a radical policy document intended as the basis for the merger that proved altogether too radical for most Liberals by recommending, among other things, sharp cuts in child benefit.
The great merger row was hugely entertaining (seen from the outside) or deeply dispiriting (seen from the inside). But, in fact, it had little to do with the SDP's fate. The truth is that the SDP had failed long before it died. And no one killed it. It was destroyed by its environment.
Only one party in British history has ever succeeded in breaking the existing duopoly of the two big parties: that was the Labour Party, when it ousted the Liberals as the main opposition to the Conservatives in the early years of this century. And Labour had three great advantages, none of which the SDP and the Alliance shared.
One was that the first three decades of this century saw a massive increase in the size of the electorate - from 7.2 million in 1906 to 22.6 million following the full enfranchisement of women in 1929. Millions of these new voters had no previous party attachment. They were clean slates on which the Labour Party could write. The SDP had no such luck. The number of voters "up for grabs" grew substantially during the Seventies - but not on that sort of scale.
Labour's second advantage was inherent in its name. It appealed directly to the manual working classes, and they not only constituted an identifiable economic interest: they happened to be the largest such interest in the country. Win their votes, and you were in. By contrast, the SDP consciously (and somewhat self-righteously) eschewed interest-based appeals to particular social groups - and anyway there did not exist a large enough economic interest to which it could appeal uniquely. Contrary to widespread belief, the party did not tilt towards the private sector. On the contrary, it appeal disproportionately to the public sector salariat (such as professors and social workers), but that section of society was not in itself large enough and, even among them, the SDP never won an actual majority.
But it was Labour's third great advantage that probably provides the key; the party that Labour sought to displace in the Twenties, the Liberals, conveniently fell apart (thanks largely to the personal rift between Asquith and Lloyd George). Perhaps more than they realised, the SDP's founders in the Eighties were banking on the Labour Party also falling apart. Labour did not oblige. Twenty-six Labour MPs defected in 1981-82, along with the Gang of Four, but the majority of the Parliamentary Labour Party - loyal to Labour, keen on keeping their seats - stayed put. Once Denis Healey had defeated Tony Benn for Labour's deputy leadership in October 1981 - signalling that Labour's left was not going to get things all its own way - it was clear that, while a limited breakaway might occur, a major split on the Labour side would be avoided.
Underlying everything, of course, was the British electoral system. It failed to turn SDP votes into seats. Because it did that, it deterred potential SDP supporters from actually voting for the party. Because it did both those things, it gave the Labour Party years of invaluable time - time to expel Militant, time to move back to the political centre, time (eventually) to find its saviour in Tony Blair.
If the SDP was doomed to die, or at least fail, what should be its epitaph? The short - and rather brutal - answer is that the Social Democrats had virtually no long-term impact. The Conservatives under Margaret Thatcher would have won their massive victories that finally led Labour to reform itself. Similarly, new Labour's policies today owe far more to the Zeitgeist of the Nineties and traditional Labour values than they do to original Social Democratic thinking.
Christopher Wren's son wrote of his father: "If you would seek his monument, look about you." Sadly, in the case of the SDP there is no point in looking around. It was exciting while it lasted, but its former supporters really have only their memories.
'SDP: The Birth, Life and Death of the Social Democratic Party', by Ivor Crewe and Anthony King, is published by the Oxford University Press at pounds 25.
What became of the Gang of Four?
She was defeated contesting Cambridge in 1987, but has sat in the House of Lords as Baroness Williams of Crosby since 1993. She divides her time between the John F Kennedy School of Government at Harvard University, where she was appointed professor of elective politics in 1988, and her home in rural Hertfordshire. Since the fall of the Berlin Wall in 1989 she has been heavily involved in Project Liberty, an American-based organisation committed to promoting democracy in central and eastern Europe. Like Rodgers, she greeted Tony Blair's election as Labour leader with enthusiasm and stressed the need for closer co-operation between Labour and the Liberal Democrats.
He announced in September 1991 that he was to stand down as MP for Plymouth Devonport, ending speculation that he might join the Major government. He praised the premiership of John Major in an article for the Mail on Sunday before the 1992 election, but took care not to endorse the Conservatives as a party. A rumour that he was to be made governor of Hong Kong proved to be without foundation. In August 1992 he was appointed the EC's peacemaker in the Bosnian conflict, a role he occupied for three years. He accepted a life peerage in 1992, but holds the House of Lords in considerable contempt and sits on the cross benches when he attends. He is regularly tipped for top jobs in public service, such as the governorship of the BBC.
He lost his Glasgow Hillhead seat at the 1987 general election, but was given a life peerage and was chosen a year later as the merged Lib Dems' first leader in the House of Lords. Also in 1987, he became Chancellor of Oxford University, appointed in preference to the Conservatives Robert Blake and Sir Edward Heath. His autobiography, A Life at the Centre, published in 1991, was lauded by the Economist as "a model of what a modern politician's autobiography should be". He has been active as a book reviewer, historian and pundit.
After failing to win Milton Keynes in 1987 he left politics to become director-general of the Royal Institute of British Architects, a post which he held for seven years. On retiring from RIBA, he became part-time chairman of the Advertising Standards Authority. In 1992 he took the title Lord Rodgers of Quarry Bank, after the Liverpool district where he grew up. After Tony Blair became Labour leader in 1994, Rodgers announced that he considered Blair the right man to lead Britain in the second half of the Nineties.
COMPILED BY SCOTT HUGHES