25 years after her election, Vernon Bogdanor looks at a former PM's ability to detect stirring issues in the loins of Sun readers

John Campbell is a political biographer of remarkable gifts. His account of Lloyd George after 1922, The Goat in the Wilderness , and his Aneurin Bevan and the Mirage of British Socialism , suggested wholly new interpretations of the 1920s and the 1940s, and threw much new light on what seemed well-worn topics. His biography of Edward Heath shares with D. R. Thorpe's recently published biography of Anthony Eden the accolade of being the finest biography of a post-war British prime minister. Campbell's achievement is the more remarkable in that the biographies of Bevan and Heath were unauthorised and so he did not enjoy the advantage of access to private papers.

The first volume of his biography of Margaret Thatcher, The Grocer's Daughter , which took the story up to 1979 and was published in 2000, equalled his previous high standard. It undermined the pieties of Thatcherite hagiography by revealing her distaste for Grantham and her embarrassment at her father's provincialism. But Campbell also revealed a great deal about his subject's early political career, showing that there was little trace of anti-Europeanism before 1979, and that she was, on the whole, an orthodox Heathite for as long as Heath's star was in the ascendant. There were indeed few signs of "Thatcherism" before Thatcher came, so unexpectedly, to be elected leader of the Conservative Party in 1975.

It has to be said that this second and final volume, The Iron Lady , is not quite up to the standard of the first. This is not wholly Campbell's fault.

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The trouble is that much of the story has already been told, not only by the protagonists - Margaret Thatcher herself, Geoffrey Howe and Nigel Lawson - in hugely detailed apologias, but also by first-class journalist historians such as Peter Riddell and the late Hugo Young. The Iron Lady does little more than provide detailed elaboration of what we know already.

Perhaps there is not much more for the historian usefully to say without access to the main primary sources - the Cabinet papers and the Margaret Thatcher archive, now stored at Churchill College, Cambridge.

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Until then, it might have been better to eschew a detailed biography in favour of a brief interpretative essay, the sort of essay that Roy Jenkins did so well in his short lives of Roosevelt and Truman. There is, it is true, a clear and coherent interpretation in The Iron Lady but it has to be extracted from a huge mass of detail; and Campbell devotes just two pages at the end of the book to an evaluation of his subject's remarkable career.

The book is far too long, and, although well written, it becomes in the end somewhat wearisome. "It weighs too much," complained Lord Beaverbrook, when offered a massive life of his fellow newspaper magnate Lord Northcliffe; and he sent it, unread, to the library of New Brunswick University. The Iron Lady, sadly, could well suffer a similar fate.

Thatcher served for longer continuously as prime minister than anyone since Robert Banks Jenkinson, the second earl of Liverpool, prime minister from 1812 to 18. She is also the only 20th-century prime minister to have given her name to an "ism". She was, as Peregrine Worsthorne noted, "the first political evangelist to occupy Downing Street" since the time of the 1945 Labour government. Yet the "ism" was invented by her political opponents in the journal Marxism Today . Thatcher proudly appropriated it for herself, and the invention boomeranged, rather as the cartoonist Vicky's discovery of "Supermac" backfired on the left in the late 1950s.

Campbell's central theme is in essence negative. There was, he believes, no developed ideology of Thatcherism, nor was Thatcher herself a particularly strategic thinker. Indeed, Campbell gives more credit to Howe, the chancellor, for Thatcherite economic strategy after 1979, than to the prime minister herself. What Thatcher provided was an instinctive understanding of the politically possible. She was able, in Chris Patten's graphic words, to "detect the first slight stirrings of an issue in the loins of a Sun reader. She puts the rest of us to shame." Thatcher and Howe thus played complementary roles in her first administration, which Campbell regards as her most successful. "Though their relationship deteriorated later," Campbell writes, "for these first two or three years of the Thatcher government, they made a formidable combination, perhaps the most successful prime minister-chancellor partnership of the century."

Thatcherism was as much a consequence as a cause of social and political change, and it was made possible by the steady decline in the size of the working class and the growing desire for home and capital ownership. Even so, Thatcher might never have reached 10 Downing Street had it not been for "the winter of discontent" in 1978-79, which made her anti-trade union message more plausible. "There is no question," declared Bernard Donoughue, head of James Callaghan's policy unit, "that the public sector unions elected Mrs Thatcher in 1979; indeed, she subsequently said thank you to them in her own individual way."

Conservatism thrives best when fear is abroad the land, and the "winter of discontent" gave rise to a grande peur that was to last until the 1990s. "Vote Labour - or else," declared two union bullies standing behind Callaghan in a Daily Telegraph cartoon in April 1979, captioned "Secondary canvassing". The psephological work of Anthony Heath showed that this fear was felt most strongly not among the managerial and professional classes, but among those belonging to what Marx would have called the petit bourgeoisie : small shopkeepers, the owners of small businesses and the self-employed. It was primarily those with small amounts of capital who, because they did not belong to the big battalions in the corporate state that Wilson, Heath (in his later phase) and Callaghan had built up, felt themselves to be the powerless victims of inflation and trade union power, and gave Thatcher her electoral majorities. Class consciousness among this group was far stronger than among the working class or the professional or managerial classes, and about 90 per cent regularly voted Conservative in the 1980s. Moreover, the vagaries of the British electoral system allowed Thatcher to win a comfortable majority in 1979 on 44 per cent of the vote, and landslides in 1983 and 1987 on just 42 per cent of the vote. Thatcherism triumphed in Britain despite the fact that nearly three-fifths of the voters were against it.

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Between elections, Thatcher's personal poll ratings broke all postwar records for prime ministerial unpopularity. Indeed, by March 1990 she had broken her previous record - that of December 1981 - when her approval rating fell to just 23 per cent, the lowest since Neville Chamberlain's premiership in 1938. As James Callaghan wrily noticed, the further away from Britain one went, the more popular Thatcher became.

She has, however, at least two great achievements to her credit. The first was to break the power of the trade unions. The second was to encourage the metamorphosis of the Labour Party into Tony Blair's "new Labour". Campbell has much to say about the former but there is little on the latter.

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From 1969, when Wilson capitulated to the trade union barons and withdrew the proposals for legislation embodied in the white paper, In Place of Strife , until 1985, when Thatcher finally defeated Arthur Scargill, governments had found themselves paralysed - not only in industrial relations, but also in areas such as education and health, by the veto of the unions. In four general elections, those of 1970, October 1974, 1979 and 1983, the threat of the veto had hovered in the background; while in the general election of February 1974, it had moved into the foreground, becoming an explicitly constitutional issue, and bringing about the fall of Heath. In destroying the veto, Thatcher restored constitutional government and ended the era of producer socialism. The Labour Party has good reason to be grateful to her.

Thatcher's final triumph, however, did not occur until the general election of 1997, when the election of Blair ensured that "Thatcherism" would survive a change of government. Labour, indeed, was seen as safe to be entrusted with power only after it had accepted the broad outlines of Thatcherism - the priority of the attack on inflation, the importance of the market, legal regulation of the trade unions and privatisation. The general election of 1997 was the first since 1918 in which nationalisation and the balance between public and private ownership were no longer an issue. The Conservatives, therefore, in Douglas Hurd's words, "lost the 1997 election, having won the arguments".

Tony Benn once declared that he had put down a motion in the House of Commons seeking to repeal every item of legislation passed during Thatcher's premiership. Yet, even if this had been passed, it would not, in Benn's view, have affected her achievement. For Thatcher, Benn believed, was a great teacher. The left, unfortunately, had lacked a great teacher since Bevan. Perhaps, however, the left had tried to continue teaching but had found that no one was listening.

The Iron Lady appears during that curious intermediate stage in history, when its subject is no longer part of the current political debate but is not yet ripe for a genuinely scholarly reassessment. For this reason, perhaps, Campbell's biography may not receive the attention that it deserves. It offers, however, an excellent interim account of her premiership and provides the raw material with which any interpretation of the last part of the 20th century will have to grapple. And yet, faced with 800 pages on a prime minister on whom so much has already been written, one is bound to feel some sympathy with those colleagues who, in 1990, declared that enough was enough, that the "ism" was turning into a "wasm", and that, as Emerson once insisted, every hero becomes a bore at last.

Vernon Bogdanor is professor of government, Oxford University.