Testing Budget for an heir apparent - his caution was so cleverly judged, it was almost exhilirating

Readers of the fashionable music magazine Word have just been treated to the unlikely vignette of the Prime Minister, with furrowed brow, trying to remember the words of "Live With Me" by the Rolling Stones. "I got nasty habits," Tony Blair said with a look of "immense concentration". "I take" - pause - "I take tea at three/Yes, and the meat I eat ..." Thirty years almost to the day after he last sang in his university band, Ugly Rumours, the Prime Minister should try to recall another Stones classic. The theme for yesterday's Budget could have been "Sympathy for the Devil".

Not that Blair regards Brown as the Devil exactly, but there are certainly those among the Prime Minister's entourage who regard the battle against the Conservatives as entirely secondary to the struggle against the real enemy next door. Friends of the Blairs who are removed from the day-to-day warfare often tell me how shocked they are by the "us" and "them" language in No 10, with "them" used to refer to Brown's Treasury rather than Michael Howard's Opposition.

Nor do I see Brown in quite such sectarian terms. I think he would make a good prime minister, but I still think Blair is better. I accept that these views put me in a tiny minority of the London liberal media class, and in a rather larger minority among the general public. There can be no disputing that opinion polls suggest Brown would be a more popular leader of the Labour Party than Blair, which is a significant shift in the terms of political trade that has occurred over the past year.

That is what underlies a more subtle change in the political landscape that has become definite only recently - the adjustment to the inevitability of Brown's succession. It was always obvious that, once Blair had put a time limit on his tenure of office, power would begin to drain from him. But for many months after his announcement in September that he would not seek a fourth term, conversations with ministers and Downing Street officials about the future continued to be about whether Blair wanted Brown to succeed him, and whether he would try to move him after the election. Brown's succession was not taken for granted.

In the past few weeks that has changed. References to what happens after the election are accompanied, metaphorically speaking, by glances in Gordon's direction. Blair could still move Brown to the Foreign Office after the election, but the assumption is that Brown would still be the overwhelming favourite in a leadership contest. The Prime Minister has the power to sack him, but that would look like such an act of unprovoked aggression for personal motives that it would destabilise Blair so much that he may not be able to survive.

That was the context for this Budget. Brown appeared to be in the happy position of riding to the rescue of Labour's much-criticised pre-election campaign, emphasising - without the need for any disloyal spin from his acolytes - what an asset his record of economic management is to the Government.

But the Budget decisions were fraught with political risk for the Chancellor. And, because he is so much the heir apparent, the risk was all on the down side. Hence the caution, a caution so cleverly judged that it was almost exhilarating. Instead of the dramatic pre-election giveaway talked up by commentators, he engaged in targeted pre-emption. He neutralised the potentially most effective Conservative ploy of the long campaign by simply buying up half the council tax subsidy for pensioners that Michael Howard had offered.

But no wonder the rest of the Budget was so predictable - and predicted. Brown was and is in an impossible position. If he had said anything remotely interesting it would have been interpreted to death within seconds for its implications for the state of Blair-Brown relations.

The fact is that Brown has not publicly said anything in disagreement with Blair for more than two years. Since then, he has been on his best public behaviour. Yes, he may have been obstructive in private. His aides were unwise, not so much to co-operate with Robert Peston's biography, but to pretend that Brown's private policy preferences were different from Blair's.

In no way am I suggesting that Blair-Brown tensions are made up by journalists, but some of the reporting of Brown's views certainly involves creative interpretation. The latest example was this week's Newsnight interview in which he said Britain should be "more than" a bridge between America and Europe. Who could disagree? Certainly not Blair. But it instantly became part of the industry of Brownology devoted to analysing the tiniest nuances of the Chancellor's utterances.

Another branch of the industry particularly active in the run-up to the Budget has devoted itself to the question of whether Brown wants Labour to win a big majority or a small one. This does not seem to be the critical issue, because it is largely out of his hands. What is much more important is the "story" of the election, which is bound to be that it is a terrible setback for the Prime Minister and that Labour would have done better had Brown been leader. Opinion polls are setting expectations that tempt many to regard a three-figure majority as the benchmark. Yet the spread-betting market was a better guide to the US election, and this week it suggests a Labour majority of 52. Comfortable, in historical terms, but bound to be regarded as a great retreat.

Yet the more Brown is entrenched as the obvious successor, the less he can risk mistakes. He is like a student predicted to gain top grades but whose exam is a long way off, and all that can happen is for him to throw away a glittering future. He deserves everyone's sympathy.

The writer is the author of a biography of Tony Blair and The Independent on Sunday's chief political commentator.

j.rentoul@independent.co.uk