For the first time, Tony Blair is lacing the darts thrown at his Chancellor with poison

The PM is acting more provocatively towards Mr Brown than is wise or politically rational

The nature of the relationship between Tony Blair and Gordon Brown changes as it deteriorates. Even in its death throes nothing remains the same for very long.

The nature of the relationship between Tony Blair and Gordon Brown changes as it deteriorates. Even in its death throes nothing remains the same for very long.

The latest dynamics are the most extraordinary yet. They are almost the opposite of what they should be. Following the war in Iraq, Tony Blair is a weaker political figure. Even his close allies acknowledge this. No Prime Minister can go to war on a premise that turns out to be wholly wrong without suffering, at the very least, a loss of authority. Yet it is since the war that Mr Blair has become more assertive, daring to take on Mr Brown in a way he has never done before. At his weakest, Mr Blair seeks to be strong.

For Mr Brown, the situation is even more contorted. As the most obvious successor to a weakened Prime Minister, he is theoretically at his strongest. A leader in waiting has an aura, an enhanced authority, especially as the current leader has given notice that he will resign at some point after the election.

Other leadership candidates come and go, but Mr Brown is always there, the clear favourite and incomparably the most formidable candidate. Cabinet ministers seeking political life after Mr Blair must take this into account. At some point in the next few years, Mr Brown will probably be their leader. Yet at his strongest, Mr Brown is pathetically weak as he faces humiliation at the hands of a newly emboldened Mr Blair.

There has been much focus on Mr Blair's petty attempt to upstage the Chancellor's speech last week on poverty by holding his monthly press conference at the same time. But there are other recent symbolic acts, such as Mr Blair's decision to hold a special cabinet meeting last month on plans for the general election at a time when Mr Brown was out of the country. Mr Blair could have waited until the beginning of this year to chair such a meeting, but Mr Brown might have been available to attend. For the first time the darts being thrown at the Chancellor by Mr Blair are laced with poison.

This is a significant new development as Mr Blair was once restrained in his dealings with his old friend. During his long honeymoon as a leader he remained largely in awe of Mr Brown. While some of Mr Blair's allies were already fuming at what they regarded as the Chancellor's disloyalty, Mr Blair tended to be more generous, still hailing Mr Brown as a "strategic genius". With Mr Blair holding him in such high and fearful esteem it was Mr Brown who tended to prevail in internal disputes although this was during a phase in his leadership when the Prime Minister could have acted as he wished.

Now a weaker Mr Blair seeks to undermine his Chancellor, enthusiastically encouraged by some of his closest allies. Almost without exception in recent months, they have been screaming for him to take on the Chancellor. Alan Milburn made it a condition for his return to the Cabinet. Mr Blair tends to form exaggerated judgements about colleagues. Once he swooned over Mr Brown's political genius, now he regards virtually all the Chancellor says and does as aimed solely at destroying his leadership.

This view of Mr Brown is wrong, dangerously wrong. There is no doubting the willingness of Mr Brown's allies to fight their corner. Mr Brown wants to be Prime Minister and expected to inherit the crown last autumn.

As I revealed in this column in September, Mr Blair and Mr Brown had several discussions about a smooth handover beginning with the famous dinner hosted by John Prescott more than a year ago. This is confirmed by the serialisation of Robert Peston's biography on Mr Brown.

Given the sensational nature of these revelations, the absence of detailed and unequivocal denials from both of them is in itself a further form of confirmation.

As I also revealed last September, Mr Blair conclusively changed his mind about resigning after the summer's local and European elections when it became clear that the Conservatives were making no headway. The failure of the Conservatives to recover politically is a pivotal factor in this drama.

Mr Blair is entitled to change his mind. He is a prime minister leading a party with a substantial lead in the opinion polls. But he is acting more provocatively towards Mr Brown than is wise or politically rational.

In some ways the Chancellor is being restrained given his recent humiliations. He has lengthy and substantial speeches on a range of policy areas that are lying in his office undelivered for fear of being seen as disloyal. When he occasionally speaks out, personal ambition is not the only factor. To take the most explosive example so far, Mr Brown has a considered view on when markets work in the delivery of public services and when they fail to do so. I do not believe he reached these conclusions solely to cause trouble for Mr Blair. He believes it.

Oddly, I have yet to read a detailed refutation from senior Blairites of Mr Brown's New Labourish arguments on the limits and the opportunities provided by markets. Instead, we hear from them ill-thought through and bland assertions about "market-based reforms".

Similarly, Mr Brown had been planning for several weeks to make his speech on Africa last week. He referred to it in some detail in an interview with me for The Independent last month. I suspect that for Mr Brown the issue is partly a metaphor for how he would approach some domestic issues if Mr Blair vacated Downing Street.

But this is not mere mischief making. Mr Brown is serious about Africa with a track record to prove it. Mr Blair is serious too. Who could not be? But Mr Blair, once sensibly tolerant of his neighbour, has convinced himself that Mr Brown must now be countered at every point. Mr Brown has been no angel in this relationship but the latest twist is the Prime Minister's persistent attempts to sideline the most formidable member of the Cabinet. A weakened Mr Blair is damaging his own more precarious standing in attempting to be so mighty.

The appalling tensions wreak havoc on the workings of the Government, but have little impact on Labour's poll ratings. Voters are used to the two of them falling out. Even so I suspect the polls are missing something fairly big. Over the holidays, I continued to meet Labour supporters who were planning to vote for the Liberal Democrats, not just trendy Londoners but all types from different parts of the country. I also continued to meet others who had met disillusioned Labour voters telling them the same story. The reason for their switch was Mr Blair's conduct as he led Britain into the war against Iraq.

For months I have dismissed such exchanges as irrelevant anecdotal evidence but there comes a point when such a persistent message acquires a wider significance. The war and Mr Blair's public statements preceding the conflict will be a more important factor in the general election than his irreconcilable relationship with Mr Brown.

s.richards@independent.co.uk