Lord Howe's memoirs are eloquent about the lengths to which Margaret Thatcher went in resisting the incorporation into the 1979 manifesto of the apocalyptic Stepping Stones document drawn up for her two years earlier by John Hoskyns. Stepping Stones made the transformation of the trade unions the fulcrum of what Howe calls the "sea change in political economy" that it identified as the task of an incoming Tory government.
Mrs Thatcher agreed with almost every word of it. But she was deeply cautious about saying so publicly ahead of the election. Even when the unforeseen events of the pre-election winter made unions the central issue of the day, she shrank from including its most far-reaching conclusions in the manifesto. Instead, as Howe says with masterly understatement, the Stepping Stones approach remained "available for guidance in the years ahead".
That isn't to say that the Road to the Manifesto which Tony Blair will unveil today masks, in any conventional sense, a hidden agenda. But it is a reminder of the shortcomings of opposition compared with the opportunities in government.
Describing Blair's passage to the election, Lord Jenkins recently offered guests at a Liberal Democrat dinner the analogy of a curator nervously carrying a priceless gossamer-thin Ming vase across a newly polished and treacherously slippery museum floor. There was an amicable criticism implicit in the image conjured by Jenkins; that Blair is too anxious about the perils of the journey, not risky enough in his impatience to complete it.
Blair doesn't see it that way; because for the Labour leader, every hostage given to a Tory press, every extravagant spending promise, every needless posture struck about issues from beef to the Prevention of Terrorism Act which his party is powerless to influence in opposition, threatens the shattering of the vase, and with it the renewed hopes of British social democracy.
So prevalent is the belief among politicians of all parties that Labour will win the general election that scarcely any of them confronts the historic meltdown it will mean for Labour if he fails to do so. Prepared to contemplate that awful prospect, Blair is at least as sensitised to the dangers and limitations of pre-election opposition as Mrs Thatcher was.
To take an example, welfare reform, and the switching of resources from social security to education, is central to the thrust of today's document. But we are unlikely to know how fast Blair intends to achieve that. And even if he had the exact blueprint and published it now, he would be engulfed in conflicting advice from every interest group in the country.
For all the populist and arresting language in which Blair is reliably said to have written today's draft manifesto, it is deliberately limited in scope.
Ian Macleod once said acidly of the Tory manifesto that preceded Labour's 1966 landslide that it contained 131 commitments but no ideas. As a result, the electorate had no notion what the party's policy was.
By contrast, the firm pledges in today's document, from the limit on infant class sizes to the promise of a job or real training place to every 16- to 18-year-old, literally can be contained in the credit-card sized document of which the party has printed two million copies. Blair has been ruthless in weeding out not only the promises that would require tax increases to pay for them, but also all those not certain of being achieved in a first term.
As late as this very week, he withstood a concerted effort to harden policy on state pensions by making explicit the goal of matching their growth to that of earnings. Earlier manifestos offered policies on what Blair once called "everything from stray cats to world disarmament". Today's document promises not the earth, but a start to Labour's aim of restoring social cohesion and economic efficiency. And although it will fail to answer some of the thorniest questions - such as child benefit for 16- to 18-year-olds and whether there will be a higher tax rate - there is no sign that the final manifesto will otherwise differ radically from today's document. For all the protestations about consultation throughout the party, the forthcoming ballot of members cannot do other than endorse it wholesale.
This is already deeply disturbing some of Blair's restive backbench critics. But it may suit the minimalist times. The message to the party is that in a climate in which politicians aren't trusted any more, it is not just pointless but fatal to promise more than you can deliver, or - and this may be even less ambitious - what the voters think you can deliver.
The party will surely absorb this message. But what it will not be able to tell from today's document is whether Blair would be as radical from the social democratic centre left as Margaret Thatcher was from the right.
First, as he reminded Labour MPs yesterday, he has to get elected. But in office there are fewer alibis than in opposition. He believes that social cohesion, welfare state reform and the re-equipping of the economy through education are as big as Thatcher's state-shrinking, union-curbing agenda. But if his momentum falters in office, then the capacity for disillusionment among the tens of thousands of party members who at first hesitantly, then enthusiastically, participated in the rewriting of Clause IV is as limitless as their hopes are now.