Labour's ruin or redemption lies with Gordon Brown

The fact is that without Brown the party's centre of gravity would start to slip, perhaps irrevocably, away from the modernisers. Blair would be exposed

What's happened to Gordon Brown? The opening of his speech yesterday fizzed with jokes. He has lost over a stone in weight, after punishing himself, typically single-mindedly, with a daily five-mile run on the treadmill of a Westminster gym. Close friends say he is suddenly more at ease with himself. Is it that he's having more fun? He is. Is it that he has got a lot of his own way in the past year? He has. Or is it that he now subconsciously realises how utterly pivotal he will be to the man to whom he traumatically yielded up the leadership in 1994? He will be.

There was a lot Brown didn't say yesterday. He said nothing about the top rate of tax and whether a new one will be imposed on those earning more than pounds 100,000. He referred to monetary union only to bait the Tories. But, by foreshadowing integration of the tax and benefits system, Brown plans to conclude the quest that Nigel Lawson reluctantly abandoned. We now know for certain that he intends to be a radically tax-reforming chancellor.

The commonplace view about Labour is that it is too timid to declare its hand about what sort of government it would be. Brown had done quite a lot even before yesterday to tell us what sort of chancellor he would be. In the process some blood was spilt, some bones broken. Brown can be charming and funny in private. He reads widely; he has a deep cultural hinterland. But he can be brutal when crossed. To still opposition to his relentless advance across the electoral battlefield, he has sometimes cajoled, as often bullied, his colleagues into line.

The ending of child benefit for parents of the 16s to 18s is a case in point. For David Blunkett, now a rising star of modernisation, this was an opportunity - not least because it put him in charge of an educational allowance to help poorer children to stay at school. Blunkett became an ally. Chris Smith, then Shadow Social Security Secretary, objected, unwilling to accept that benefits for low income groups should be financed by removing them off the better off. Smith was summarily switched and replaced by Harriet Harman. The proposal was Brown's own. Blair backed it, but only after carefully weighing up the real electoral risks. The big advantage was that it helped to prove, first, that Brown was serious about containing spending. It was bold proof that ministers would have to save before they could spend. And, secondly, there was more than rhetoric to Blair's stated goal of reforming the welfare state. There had been a lot of warm words about tough choices, but this was the real thing. A dam was breached. Suddenly, it no longer looks so inconceivable that a Labour government might tax child benefit for higher-rate payers.

Brown's constant desire to give himself space as chancellor is apparent in the pre-conference deal he brokered on pensions - a deal that leaves intact Labour's freedom to channel increases in pensions to the poorest rather than those who don't need them.

But it is evident, too, in his position on monetary union, that Brown is not as doctrinally committed to EMU as Kenneth Clarke. He has a deep sense of the potential of staying out. That's why, ideally, he would like a manifesto wording on EMU sufficiently in favour to allow a Labour government, if necessary, to claim that electoral victory gave it a mandate and that a referendum is unnecessary. He recognises that all the experience of previous Labour governments is to postpone difficult decisions until it's too late -the IMF crisis of exactly 20 years ago being the outstanding example.

A temporarily unpopular Labour government might not win a referendum, especially one fought out against an anti-EMU Tory party. Brown's toughness, therefore, isn't in doubt. He is a driven politician, more so in some ways than his leader.

You can quite easily imagine Blair leaving politics in the event, scarcely thinkable in the optimistic atmosphere of Blackpool, of Labour suffering its fifth defeat. It's almost impossible to imagine Brown doing so. And if you doubt his importance, simply consider for a moment a Shadow Cabinet without him. Robin Cook may dispute what he was reported as saying about the need not to sacrifice the poor to the desire for votes in middle England. But he thinks it - and more. Neo-Keynesian to the last, he is the leading spokesman of the left. Blair needs him, too, not only because of his abilities but because of the constituency he delivers in the run-up to the election. It is fortunate that Cook and Brown, the two cleverest men in the Shadow Cabinet, get on better with Tony Blair than they do with each other. It's fortunate, too, that for all the weekend tremors Cook is as single-minded about victory as his colleagues. But the fact is that without Brown the party's centre of gravity would start to slip, perhaps irrevocably, away from the modernisers. Blair would be exposed in a way it would be impossible to imagine him being when he stands up to speak this afternoon.

Brown also knows how to translate the tough messages of modernisation into a language that the party can understand, has spoken for decades. It's apparent in the case of child benefit and pensions, where Brown has started to challenge, in terms distinctively of the left, the assumption that it is possible to have at the same time both universal benefit and redistribution.

Brown has started to show that ending some universal benefits can be a means of redistribution. Whatever its other virtues, it doesn't underpin social justice to - as he put it yesterday - "justify the wife of a millionaire receiving child benefit for a teenager over 16 when the mother of an unemployed teenager does not".

But it's apparent, too, in his familiar warning yesterday that rising inflation hits the poor hardest, or in the invocation of Aneurin Bevan to preach the tough language of priorities. Those in the conference hall yesterday who worry most about where Blair is taking them do not love Brown. But even they recognise the depth of his anger about unemployment. And they are beginning to recognise his central message yesterday that drastic tax cuts for the working poor can offer more hope than increases in demeaning benefits. They also know that he belongs to the tribe in a way that they are not quite sure that Blair does. It's fair to assume that after the leadership trauma of 1994, it can never quite be glad confident morning again for the Blair-Brown friendship. But it is indispensible for what Blair wants to do in power. Still the closest, it is now also the most important relationship in British politics.