Donald Macintyre: Mr Brown could still reach No 10 - if he is patient

'Blair and Brown are Labour's greatest assets, but their dysfunctional relations also the greatest liability'

Political succession is a fraught question in a system without limited fixed terms. After 10 years, Margaret Thatcher discovered that there isn't much space between the rock of saying you will go "on and on" and the hard place of being a lame duck Prime Minister by envisaging your departure.

Tony Blair, of course, has nowhere near reached that point yet. When he rises to address the TUC today it's safe to assume his eventual departure will be the last thing on his mind. Nevertheless, yet another book about New Labour – this time The Rivals, by James Naughtie, about Mr Blair and Gordon Brown – has put the question, however distant, back into the ether. According to the first serialised extract, Mr Blair has firmly decided to fight a third election as leader. And that, he implies, is bad news for Mr Brown's hopes of succeeding him.

Naughtie, who knows both men well, gives a telling portrait of how this deeply fraternal relationship – at once extremely intense and extremely informal – is seen within government, down to Mr Brown's habit of immersing himself in his papers, and sometimes writing furiously, during Cabinet meetings. He describes the remarkable duality, a "kingdom divided", of a leadership in which Mr Blair is apt to tell ministers that he will "clear" decisions with the Chancellor. He could have mentioned too that ministers and officials sometimes talk, in delineating the tacitly agreed boundaries between the two men of "Brown departments" (Environment and Transport, Trade and Industry, Work and Pensions) and "Blair departments" (Education, Health and the Home Office). Or Mr Brown's suggestively distancing, though no doubt unconscious, habit in joint strategic discussions with Number 10 of saying "you" could or should do this rather than using the more collegiate and normal "we".

Naughtie is also convincing on the accord which was designed to alleviate the deep hurt and frustration inflicted on Mr Brown by his honourable decision to stand down in favour of Mr Blair in 1994. This was partly a matter of degree of the autonomy afforded to Mr Brown – and similar to that demanded by Macmillan when he accepted the Chancellorship from Eden – on issues of economic and social policy, but also touched on the radioactive issue of the succession.

As Naughtie sensibly accepts, the exact nature of that discussion remains a mystery. But he credibly suggests that Mr Blair will have laid emphasis on his desire not to court the troubles visited on Mrs Thatcher because she stayed too long. Given Mr Blair's famous talent for making his interlocutors feel he has told them what they want to hear, it would be understandable if Mr Brown had inferred that Mr Blair expected not to stay in office for more than two full terms.

But even if that were the case, there were two problems. First, a general, unwritten assurance of this sort could hardly have been bankable, as Mr Brown would know. That is one reason why, according to some accounts, he has this year vainly pressed Mr Blair once again for a putative departure date. But secondly, Mr Brown's ambitions for the future went hand in hand with a strong sense that that he would almost certainly be a much better Prime Minister. There isn't anything historically unusual, let alone ignoble, about this among the most capable and senior politicians. Most, however, have for reasons of self-interest as much as any other taken more trouble to disguise their feelings than does Mr Brown.

The euro isn't the only issue which has the potential to be vitiated by these tensions. But it is the likeliest, and certainly the most important. Naughtie refers to an occasion one Wednesday afternoon before the election when an official was "knocked aside" by Mr Brown racing through the door to begin an angry discussion on Europe. My guess is that this was on the day of the Prime Minister's Question Time when Mr Blair announced that the economic tests for EMU membership would be completed within the first two years of a new parliament. Mr Brown was certainly furious, and sought out Mr Blair to tell him.

But then that was one of the relatively rare occasions when Mr Blair departed from a careful script agreed with Mr Brown. At present that script appears to preclude any attempt to influence events – rather than merely await them – by beginning to argue more consistently for British Economic and Monetary Union entry in principle. Formidable as are some of the arguments at his disposal if he chooses to oppose entry in circumstances in which the Prime Minister may want it, it is hard to disentangle Mr Brown's power of veto from his own ambitions for the succession.

It's clear that the euro question gives Mr Brown powerful leverage in his dealings with the Prime Minister. Whether it is necessarily in Mr Brown's own interests to exercise his veto is a more complicated question. In one supposedly benign scenario, Mr Brown would agree to support Mr Blair by giving him his referendum in return for Mr Blair standing down after it is over. The problem is that politics is almost always messier than that. Particularly if Mr Blair now wants a third term. What isn't at all clear is that this, if true, would in itself somehow dash Mr Brown's hopes of succession.

Of course there are other possible contenders: the Charles Clarkes, the Blunketts, the Milburns. But Mr Brown is already indisputably one of the biggest figures in post-war British politics. He has the greatest intellectual range and reach in the Cabinet. Most of the arguments against his candidacy can be immediately dismissed, such as the idea that devolution has made it impossible for a Scot to become Prime Minister. Not only is this a constitutional outrage; but it is much more plausible to imagine that a Scottish Prime Minister might be needed to save the Union. Nor is his authority so slenderly based that it would necessarily be undermined by a serious economic downturn.

The timing and form of a Prime Minister's departure can carry great influence over the succession, as Mr Blair knows well. In a more orthodox world that ought to smooth relations between the incumbent and his best-qualified successor rather than exacerbate them. The paradox of Mr Brown is that no one has a greater strategic sense of purpose, or of the big issues in politics, than the Chancellor; few are more sensitive to trivial slights that could be perceived, however wildly, as undermining his well-justified claim on the succession.

For now, the twin presence of Mr Blair and Mr Brown is New Labour's greatest asset. But the frequently dysfunctional relations between them are its greatest liability – and still have the potential to end in tears. It is hard at present to see that would be in Mr Brown's interests any more than in Mr Blair's. Lord Callaghan reminded Mr Brown in a letter of sympathy when he stood down in 1994, that he, the older man, succeeded Wilson in 1976. Whether or not it's true that Mr Blair now firmly intends to lead Labour in a third election, the Callaghan letter suggests that thequality Mr Brown yet needs to discover in himself is patience.

dmcintyre@independent.co.uk