Mr Brown is by far the best qualified man to succeed Mr Blair. But when?

The Chancellor needs to watch out that his loftiness isn't seen as self-serving as well as magisterial

Last weekend, when Mr Blair was enjoying one of his worst press onslaughts since the election, the famous government Media Monitoring Unit received a request from the top Treasury team accompanying the Chancellor of the Exchequer at the G8 summit in Canada. Could they provide the main cuttings on the saga of Mr Blair and the royal lying-in-state? Whatever the reasons for the request, it wasn't a preparation for Mr Brown to weigh into the argument with a detailed defence of his long-standing colleague.

Indeed it has, not for the first time, become a matter of common gossip that the Chancellor has been conspicuous in his absence from a big bad battle in which the Prime Minister is embroiled. John Reid, Jack Straw, Margaret Beckett among Cabinet colleagues went promptly and robustly onto the airwaves in Mr Blair's defence, whatever private doubts they may have about the wisdom of Downing Street's complaint to the Press Complaints Commission. So did a number of MPs, some distinctly politically unsympathetic to Blairism. From the Chancellor? Not a word.

On the one hand, this neatly illustrates Mr Brown's magnificent exaltation above the fray of the squalid tittle-tattle of soap opera politics. On the other, it illustrates a fairly chronic tendency not to see the normal collegiate requirements of collective leadership as applying to him. Whether the first will help him to become the next leader of the Labour Party more than the second will hinder him is now becoming one of the most interesting questions in politics.

It isn't difficult to construct a scenario in which Tony Blair ceases to be Prime Minister before the next general election and Gordon Brown succeeds him. The Labour Party is probably less sentimental about Mr Blair than it has been about any previous leader since Clement Attlee. Inspiring as he can be as a speaker, Mr Blair doesn't make them go weak at the knees or misty-eyed at party conferences. His huge attraction to the party since 1994, however, is that he has been an electoral killing machine unparalleled in the party's history.

But suppose that ceased, or seemed to cease, to be – a prospect which looks all the more possible after the hammering Mr Blair has received in the past fortnight. Maybe Labour will, finally, manage to lose an important by-election. Maybe by this time next year, or even earlier, Labour will fall behind in the polls. Would Mr Blair have not outlived his usefulness? Particularly if that same declining popularity made a euro-referendum – a central part of his project and one over which Mr Brown holds great sway as the custodian of the economic tests – manifestly less achievable, thus robbing him of something all prime ministers need: momentum. Would not the way of guaranteeing another victory be to switch leaders and opt for the man whose poll ratings are soaring and who has seen himself as having the prior claim since John Smith's death eight years ago?

Well, maybe. It is certainly easier than it was to find MPs and ministers prepared in private to contemplate the possibility. But there are some rather important countervailing factors.

The first is the reaction of Mr Blair himself. Neither he nor his wife – the recent target of a fierce press onslaught for what was actually a quite human statement of the obvious in relation to the Middle East – show any sign of simply wanting to give up the ghost. On the contrary: just as the concerted effort to oust Alastair Campbell from his job last week made it less, rather than more, likely that he would go, so it is with Mr Blair. Because he has had it so easy for so long, it's hard to remember that he also has the capacity to take risks and to fight when up against the wall. And, for what it's worth, there isn't much sign – for all the conventional wisdom that it may well now prove impossible – that he has given up on the prospects of a euro-referendum. As his reasonably robust remarks in the Commons indicated yesterday.

Mr Brown also has yet to win over support from much of the Cabinet. There was an electric little moment at the Cabinet last week when Mr Brown – very unusually speaking outside mere economic matters – dwelt on events this week in Parliament, including a Tory attempt to suborn the campaign for development aid. The unspoken contrast with Downing Street's perceived obsession with press matters was obvious. After a few grumbles from John Prescott about the recent Clinton-Blair (and indeed Brown) seminar, Mr Blair pointed out that the Government had a huge majority, a very strong economy – thanks of course to Mr Brown – but history showed that previous Labour governments had fallen apart because of personality splits. No one demurred.

The second is the impact of Mr Brown's behaviour on the party itself. Of course many in the party would find the leadership of Mr Brown, culturally much more a Labour heartlands man than Mr Blair, more congenial. John Prescott is rather typical of the mainstream; his genuine closeness to Mr Brown has recently been cemented by the promise of a generous spending settlement. But Mr Prescott would not tolerate the replacement of Mr Blair before he is ready.

There are two sides to Mr Brown. One is the absolutely alpha, commanding, identifiably left-wing politician, warm, witty, intellectually as brilliant as he is curious, the kind of man whose conversation on an impossibly wide range of subjects will grip you until the end. The other is a politician of high impatience, who will – as he reputedly did recently – complain that Mr Blair, rather than he, will be giving the keynote speech to the TUC Congress this year. (Mr Blair's address last year was aborted by the attack on the World Trade Centre.)

For all its warmth towards Mr Brown, the party doesn't much like attacks by the Tory press on its leader. Loyalty and unity are pretty deeply ingrained in the Labour Party. So too is gallantry, of the sort Kenneth Clarke regularly used to show to John Major, even when he most wanted his job. Whatever their faults, Mr Campbell, who is positively old Labour about, say, education, and Cherie Booth, whose remarks on Tuesday will be welcomed by many Labour MPs, also reach quite far into the party in Parliament in the country, too. Mr Brown needs to watch out that his loftiness isn't seen as self-serving as well as magisterial.

Mr Brown is far and way the best- qualified man to succeed Mr Blair. He has every claim to be the best Chancellor since the war. He is one of the most significant British politicians of the last 100 years, with a towering grasp of the sheer business of government. He is arguably the principal architect of the modern Labour project. More than that, he has a driving sense of what he wants to do with power. Nor is he always wrong in the continuous and often acrimonious disputes he has with No 10. But, contrary to what he sometimes to seem to think, he may endear himself more to the party that will elect him by coming to its aid when it is in trouble than by staying permanently aloof.

d.macintyre@independent.co.uk