The great SDP project I failed. The mould of British politics I remained unbroken." That is the conclusion of this remarkable and detailed history of the Social Democratic Party, which ends with the general election in June 1987, and the failure for the second time of the SDP/Liberal Alliance to win more votes than either of the two old political parties.

In the sense that the Conservative and Labour parties still dominate national elections, and the media coverage of those elections, the authors are right. Neither the SDP, nor the Alliance succeeded in their original goal of replacing the Labour party. The main reason for that was the first-past-the-post voting system (and the party diarchy that ensures that the system survives). The Alliance, while winning 25.4 per cent of the vote in 1983, and 22.5 per cent in 1987, never held even 5 per cent of the seats in Parliament. It is remarkable that its vote held up as well as it did, given the "wasted vote" argument. All this is made plain by the Ivor Crewe and Anthony King.

This study of the SDP is a classic of its kind, accurate, detailed, perceptive, mercilessly honest. It is a study firmly based on psephology. The book gives us detailed profiles of those who voted for the SDP, those who were its candidates, those who joined it, where they lived, and much more. It shows that the SDP attracted people who had not been involved in politics before, and who came from what might be called the Robbins generation: men and women from modest backgrounds, whose parents were blue-collar workers or held clerical and lower managerial jobs.

They had benefited from the higher education now open to them for the first time ever. Many of them worked as professionals in the public services or in jobs associated with the growing information revolution. They were members of the meritocracy, disciplined, hardworking and innovative. And they did not care at all for the wheezy, cosy, incestuous and out-of-touch political system they had inherited.

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This profile of the SDP accords with my own experience. I have, however, two objections to the authors' conclusions. The first of these goes back to the principle of the merger argument itself. Those of us who supported the merger between the SDP and the Liberal party are surprised to be told that our party has died, been buried and even earned itself a distinguished obituary, these last rites being conducted by the authors themselves.

This is indeed the view of history held by David Owen and his close colleagues, who embarked on the hopeless enterprise of turning their angry faction into a viable fourth party. The SDP, in form, did die with merger, or rather, did so two years later, when Dr Owen gave up the resistant rump he had endeavoured to sustain with a minority of supporters. That has the convenience for the authors of offering, at least in form, the semblance of a finished story.

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But it is not the view of those of us who carried the majority of the vote cast by SDP members in 1987. We now belong to a combined party, the Liberal Democrats, making impressive progress in local government, and playing a key role in constructing a reformed political system in Britain, as ambitious a project as that of the Liberal reformers a century ago. The Liberal Democrats are, to us, integral successors to the SDP/Liberal Alliance, continuing, under another name, the thrust and purpose of the original SDP. The authors do not contest our view. For the sake of their claim to completeness, they simply ignore it.

And that leads me to the second objection. The authors claim towards the end of their book that nothing has changed. "It is a measure of the British party system's resilience, and of the power of the first past the post electoral system," they write, "that the most serious challenge to the system in half a century ended in such failure, making no discernible impact." And here I believe they have made the mistake implicit in their own methodology.

The traditional parties remain in name the same as they were. The votes cast for them have declined a little, but not dramatically. But the authors' metaphorical traveller, who lived abroad throughout the 1980s, and on his return in the mid-1990s found that nothing much had changed, must have been amazingly unobservant.

Surely he must have noticed that the Labour party's politics, as adopted by the party conference in 1995, bore a remarkable resemblance to those of the SDP in 1981 and very little to those of the Labour party of that era. Surely he must have noticed the dramatic decline in trade union membership and trade union influence. Surely he observed the return of a very rare phenomenon, Conservative MPs willing to vote against, and to refuse the Whip of their own party. Surely he would have been surprised to see the Law Lords, long inured to the habit of compliance, upholding the rights of individuals against ministerial decisions, and even supporting the consolidation of the European Convention on Human Rights into British law. The mould may not yet be broken, but the cracks are visible.

It would be absurd to attribute these changes to the SDP alone. Nevertheless the authors write: "Under Neil Kinnock's leadership after 1987, Labour replaced the rhetoric of class with the rhetoric of citizenship and adopted as its underlying theme, ironically, the social market economy. On price and import control, a wealth tax and unilateralism, the party executed a 180 degree turn." The new programme, say the authors, "was virtually indistinguishable from the SDP's programme in 1987".

But all this, they contend, "owed almost nothing to the SDP". The reasons offered for believing this startling statement are remarkably weak. One is that most of the changes occurred after 1987, "by which time the SDP had effectively ceased to exist". But by then the Liberal Democrats had come into existence, and quite rapidly established their party as a challenger to the other two parties both in local elections and in by-elections. In the former it did better than its predecessor, the Alliance.

Far from having "ceased to exist", the merged party, after a rough and contentious infancy, had by 1990 plainly survived and was indeed well established. It is only by clinging to their own somewhat perverse definition of the SDP as consisting only of the anti-merger Owenites that the authors are able to prove to their satisfaction that the party had no influence on Labour's changes.

While I do not share the authors' conclusions, I acknowledge and appreciate the professional quality of their achievement. Their historical reconstructions are remarkable, both for readability and for accuracy. Their account of the dispute over merger, towards the end of their story, is painful for a participant to read, but authoritative within my own range of observations. At the start of the story, what they have to say about the SDP's origins accords with my own recollections, and is written with insight and verve. They are right about the genesis of the SDP, and about the motives and objectives of its founders.

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I believe the SDP/Liberal Alliance, and its successor, the Liberal Democrats, have had a significant influence on the new Labour party, and will continue to do so. The function of offering an alternative home for left of centre voters and perhaps also right of centre voters acts as a moderating force on both the traditional parties. To potential Labour voters it is a guarantee against Labour moving once again to the far left. To potential Conservative voters it offers a commitment to the values of individual liberty and social harmony that marked the honourable one-nation tradition from Disraeli to Butler. The Liberal Democratic party, as the SDP's successor, remains a pioneering party, moving into the minefields of tax and expenditure policy, to determine whether the electors will really support with votes what they claim in opinion polls. It probes our bipartisan status quo, a calcified monument that will crumble if it is not reconstructed.

As the authors point out, the Gang of Four had its greatest differences over relations with the Liberal party. It was obvious at a very early stage to most of us that to fight one another, given the first past the post system, would be suicidal. The search for some basis of agreement on an electoral pact occurred at a very early stage, and was, as the authors indicate, the basis for David Owen's later belief that there had been a conspiracy to hand the infant SDP over to the Liberals.

This, as I can attest, was total nonsense. Neither Bill Rodgers nor I had had any close dealings with the Liberal party, nor did we know its leaders well. We regarded it as amiable and confused, but led by a succession of remarkable politicians who tried and failed to reconstruct their individualistic and idiosyncratic party. Jo Grimond had inspired it, Jeremy Thorpe had entertained it, and David Steel had distanced himself from it. Modern-minded, brilliant and realistic, he was in many ways a Social Democrat.

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Bill Rodgers and I saw much more of the Liberal party on the ground than did David Owen. Bill had to deal with hard-nosed and streetwise Liberal activists, who felt they knew their constituencies from the cracked pavements upwards, and who scorned psephological and local polls. Yet tough as the negotiations on the allocation of constituencies turned out to be, Bill had a certain respect for the sheer stubborn persistence of these men and women, sticking with their party cause election after election.

As for me, doing my job as SDP president, attending Liberal conferences and taking part in local campaigns, I observed the gradual transformation of the Liberal party, especially after the 1985 and 1986 elections. It was a transformation that owed something to the influence of the SDP, but much more to the experience of winning office, and learning the discipline of making decisions at city and county level.

The Liberal Democrat party of the 1990s has some of the ideals and aspirations of the historic Liberal party, especially on issues of individual rights, decentralisation of power, dislike of the influence of organised interests, enthusiasm for partnership between employers and employees; but it is a different party both in style and substance. It is also different from the SDP, especially the SDP of the Owen ascendancy, which began to resemble a cult.

David Owen spent little time with Liberals on the ground. His interaction with the Liberals was with the parliamentary party, a party whose MPs had survived by winning against the tide, by putting their constituency interests first, last and always. Party discipline was almost unknown, and the party allowed MPs to "vote the district" if they needed to.

To a leader with David Owen's authoritarian cast of mind, this indiscipline was intolerable. He himself was almost singlehandedly keeping the SDP alive, a phenomenal achievement. He did not see, perhaps could not see, the gradual change in the Liberal party, starting at the bottom in the district councils, nor the lessons experience was teaching many of its members. His own frustration with the Liberal parliamentary party coloured his opinion of the party as a whole.

David Owen's political style closely resembled that of the prime minister, Margaret Thatcher, whom he greatly admired. On the Falklands war, the British nuclear deterrent and the miners' strike, his views, and, more important, his instincts, were very close to hers. Like her, he found many of his colleagues contemptible, and he showed it. Personally generous, capable of being both an inspiring leader and a loyal subordinate, David was incapable of working with those who regarded themselves as his equals. Yet that was what the Alliance required.

Roy Jenkins is described by the authors as wanting very much to be prime minister: they quote a journalist's description, "an old man in a hurry". That does not really describe the Roy Jenkins I knew as a colleague in the Gang of Four. He would have liked to be prime minister, but his career demonstrates that on more than one occasion, he cared more for principle than position. In particular, his resignation as deputy leader in 1976 showed how reluctant he was to lead a party he no longer believed in. He never subscribed to the political tactic of going along to get along. But he did want to be leader of the SDP, and the Jenkinsites wanted it even more than he did. They were "plus royaliste que le roi".

He was not, however, the right leader for the times. In his absence abroad, politics in Britain had become savage and crude. The recognition that certain values were shared between the parties' leaders, values of personal liberty, civic behaviour, even of social justice, provided common ground between a Roy Jenkins and an Iain Macleod, a Robert Carr or a Willy Whitelaw. By 1981, that common ground had been destroyed by the fundamentalism of both right and left. The prime minister conceded nothing to her opponents, and her opponents tried to reply in kind. The courtesies had gone.

When Roy rose to ask a question, the benches around and behind him erupted with heckling and derisive laughter, the benches in front of him with ill-concealed exasperation. Four years as president of the European Commission had not prepared him for such barbarities. He hated it, a civilised man in an uncivilised place. His failure as leader of the SDP says more about British politics in the 1980s than it does about Roy Jenkins himself.

In these respects and a few others, dotted here and there in this accomplished narrative, I quibble with the authors of the book. In the two major respects mentioned above, I differ with their conclusions. That said, theirs is a notable accomplishment, valuable as history, stimulating as contemporary commentary, detached as befits two fine political scientists without being "academic", and throughout its many pages, a delight to read.

I happen to believe that in 20 years' time, they will reach different conclusions; but if I am right, and years from now people regard the birth of the SDP as an important milestone in political history, they will acknowledge that it was fortunate indeed in its chroniclers.

Baroness Shirley Williams was president of the SDP and is now professor of elective politics, Harvard University.