If Blair makes it, there will be many such days ahead. To govern, in our times, is to struggle to hold your course on a turbulent sea of sudden economic events, under constant criticism from the public and press, and with fewer effective powers than any earlier generation of democratically elected leaders.
Blair must be acutely aware of how difficult it would be for him, in office, to avoid the plunging poll ratings and party denunciation that John Major has suffered. Although their mutual coldness is turning into warm dislike, the two men share a deep pragmatism, and the question that confronts Blair now is how to construct a policy programme that gives him a racing chance of being judged a successful prime minister where Major has failed.
It won't be easy. Tories and left-wingers alike look at the huge expectations building up, the hunger for taxpayers' money displayed by a hundred lobbying groups, and they conclude that a Blair government is destined to fail. However determined he seems now, they mutter, he is fated eventually to betray the hopes that he so emotionally raised on Saturday. The numbers don't add up. They can't. They never will.
Yet Blair and Gordon Brown have already accomplished the first part of that task, the ruthless demolition of unrealistic expectations. A Labour government committed to swingeing tax rises on companies and the middle- classes, massive re-nationalisation, substantial pay rises for public- sector workers, and the swift abolition of unemployment would certainly have condemned itself in advance to betrayal, fiasco and U-turn. Now no one in the country seriously thinks New Labour would try any of that.
The second part is to produce policies that are popular, fundable and distinctive. This is rather harder - especially the distinctive part. Come the next election, both big parties will try to persuade us that they are offering wildly different prospectuses on tax, on Europe, on public services. And they will be wildly exaggerated.
The global economy has narrowed the freedom of any government to slash taxes or to raise them sharply. If taxes are unusually high, corporate investors and those higher-paid people who can, simply leave. And if taxes fall below a certain level, the country is unable to provide the educational and other support a developed economy needs. Labour or Tory ... they both have to conform.
Within this small room to manoeuvre, Labour has different priorities from Kenneth Clarke - cutting inheritance tax will never be a Labour priority. Gordon Brown is more committed to progressive taxation than his opposite number, and more hostile to indirect taxes. More generally, Labour wants to take a different approach to the Bank of England, to high-street banks, to monopoly law, to dividends, to training. Over time, by which I mean five years or more, there would be a different style of economic management that could be set against the Tory record and judged. Over time, better- off voters would pay more. But to begin with many voters would notice relatively little difference. Indeed, if Clarke introduces pre-election tax cuts targeted on poorer voters, as he is being privately urged to do, Labour might very well vote for them in the Commons.
Beyond economics, the most important areas for Labour to explain itself more clearly on are health and education. Major has succeeded in marking out a new model for the delivery of public services, based on a culture of management and measurement, with a emphasis on clear contracts rather than Whitehall bureaucracy, and on giving consumers power through information - league tables for schools and hospitals, performance targets, audits (endless audits).
This management doesn't threaten the size of the state, to the distress of the Tory right, but it helps rein in producer control over public services. Would Labour eradicate this culture?
The party mocks the Citizens' Charter and has criticised the crudity of Tory league tables. But Labour is keen to advertise the success of its own, original versions of the charter. Everywhere, too, it speaks the language of audit and value for money. It would not remove information for users of services. It can, and no doubt will, change things at the margins, slowing down the NHS changes and ending compulsory competitive tendering. But, as with tax, the real distinction over public service provision would take years to become apparent.
A final example. Major affects outrage at Blair's Europeanism. But how much does Blair's promise that a Labour Britain would not be "isolated in Europe" differ from Major's never-disowned declaration about being "at the heart of Europe"? There will be differences, such as over majority voting in some areas and, perhaps, the single currency, one day. These differences will be exaggerated if the Tories lose the election and swing more violently against Europe. But Labour's Robin Cook has no more intention of allowing his party line to veer into federalism than Douglas Hurd does.
I am not saying there would be no difference between Blair and Major, still less that the former is a closet Tory. The two are utterly divided by Blair's commitment to real political reform, which sounds stronger day by day, and by other specific policy differences, such as Trident, rail privatisation and the minimum wage.
But I am saying that Labour's claim to have distinctive, better programmes across domestic policy is something that would take years to establish and test. Eventually there would be a different tax structure, different controls over hospitals and schools, a different government approach to business. But there appear to be no quick fixes on offer, no dramatic changes in ownership, no lurches in public services.
This leads one to wonder how a Blair government hopes to get through the hard times of the first years, when its proposals for Home Rule, reform of the Lords and so on, are battering their way through Parliament, but when public-sector workers, parents and patients are still waiting to see any real changes.
Here, finally, is where the events of the weekend do matter for Labour's chances in office. As Blair hinted broadly, the internal reforms are far from over. I would expect not only changes to the National Executive Committee but to conference, too. Their aim will be to try to ensure that a Blair government is supported by a movement of its political friends in the country - what one calls "a lay establishment" - not quickly and bitterly attacked, as previous Labour governments have been, by hostile activists shouting betrayal and trades-union leaders claiming ownership rights over struggling ministers.
As he acknowledged emotionally, Tony Blair is creating a new party to which ten of thousands are flocking. What he doesn't know is how many are fair-weather friends, and how many understand the length and hardness of the road ahead. Of Saturday's jubilant faces, how many would still be with him, loaning their trust, patience and understanding as he sat, tense and harried on that difficult afternoon in No. 10?