John Rentoul: It was bound to end in tears and now we know just how bitterly they were shed

"It's a very complicated relationship," said Sally Morgan, the Prime Minister's former gatekeeper, last week. Well, that is one way of putting it. Tony Blair and Gordon Brown were once a partnership, then a rivalry and now they are a sequence. The history books will go Blair, then Brown. And it is now undeniable that it is not a sequence that Blair wanted.

The main complication of the relationship, therefore, is the difficulty the rest of us have in understanding how it survived for so long without breaking apart entirely. Baroness Morgan also said in her intriguing interview on BBC radio that the two principals are the only people who really know what went on between them. That is true; Blair and Brown are the only people who can describe the internal emotional architecture. Only each man can say for himself whether the word hatred, loathing or contempt fits best.

Yesterday, Barry Cox, an old friend and neighbour of the Blairs, said on television that he had a conversation with Cherie after the 2001 election "about how difficult Brown had become and she was demanding he resign then. And ever since then, it has been continuous."

Cox is a reliable witness. I still remember him telling me, a few months after it happened, how he had bumped into Cherie at Holborn Tube station on the day John Smith died, 12 May 1994. She was on her way to Heathrow to meet Tony, who was returning from Scotland, and wanted to make sure that he stood up to Gordon. "Cherie was worried because Tony had always had this view that he shouldn't run against Gordon," he said.

The real significance of that conversation was that Cherie needn't have worried, because Tony saw that the Labour leadership was in his grasp and ruthlessly demonstrated the truth of Lloyd George's maxim that there is "no friendship at the top". There is, however, ego management at the top, and Blair managed Brown's with some skill. He kept him inside the tent - "inside the tent spitting in," as one Blair aide put it to me - and set the scene for the 13-year psychodrama that followed.

There was just one problem for chroniclers of the Blair years. We knew that Blair and Brown were like "scorpions in a bottle", as the proxy battle between Peter Mandelson and Brown was once described, but the evidence was all hearsay. It could always be conveniently dismissed, as Cox's testimony was by Brown last week, as only "rumour and gossip".

Well, now we have the documentary proof. As Marie Woolf reports today, this week's handover to Brown is what Blair sought to avoid. Now we have it in black and white that Blair wanted to move Brown from the Treasury, just as Cox said Blair had told him. The plan to move Brown, drawn up before the 2005 election campaign, was part of Blair's wider strategy. It followed on from his declaration at the end of the 2004 Labour conference that he would put himself forward for a "full third term" but would not then stand for a fourth election. That pivotal moment of his decline and fall has been widely misunderstood, not least because Blair himself had to pretend that it was a surprise announcement issued from a position of strength, rather than a defensive tactic designed to compensate for his weakness.

Blair's problem with Brown was not that the Chancellor was plotting against him - although he was, while always being clever enough not to get caught or to do anything overtly disloyal. What drove Blair to distraction and infuriated his staff was Brown's refusal ever to volunteer his services in the Prime Minister's cause when crises broke. Brown would only ever speak up for Blair when his failure to do so had begun to be an embarrassment - although he always did so unambiguously when the moment finally came.

Blair's real problem with Brown was that Labour MPs had had enough of the most successful leader they had ever had and were beginning to think that they might stand a better chance at the 2005 election if Brown were in charge. Iraq had taken its toll, especially as conditions there continued to worsen. It also took its toll on Blair - although, as Lady Morgan said, it was not the original charge of deception that got to him, it was that "Hutton had cleared him on those counts in such an obvious way, but then when the thing didn't move on, he found that difficult".

By putting a time limit on his leadership, Blair hoped to head off the gathering revolt, which was being incited by the endless media speculation about when he planned to go. And he succeeded. Even when, in January 2005, an NOP poll for The Independent suggested that Brown as leader would stretch Labour's margin over the Conservatives from eight points to 21, the rumblings were muted (partly because an eight-point lead was enough for a comfortable victory).

The next stage of Blair's strategy was less successful. By fending off the Brown succession, Blair hoped to create the space for an alternative leader to emerge. Hence the restoration of Alan Milburn to the Cabinet in September 2004, in charge of the election preparations, and Blair's refusal to appoint Brown to Labour's National Executive. Charles Clarke, made Home Secretary in December 2004, was the other serious possibility. And hence, too, John Birt's blueprint for getting rid of Brown and splitting the Treasury.

But it was not to be, and the reasons why not are fascinating. It was not so much a matter of who hated whom, of plotting and manoeuvring, although there was a lot of that. It was decided by what Brown would call "the fundamentals" - particularly as they were reflected by those opinion polls that Blair sought to finesse. As the 2005 election approached, the Chancellor remained an insurmountable electoral asset. Like the finance director who ends up in the top job because he is the only person who understands double entry bookkeeping, Brown's keeping of the nation's books in good order paid off. As a result of his standing in the country, Brown had such support among Labour MPs that it would have been madness to sack him. The unhappiness with Blair would have found a focus and he would have been out within days.

Philip Gould, Blair's pollster, had the numbers. Alastair Campbell, Blair's former press secretary, had the political horse sense. They joined forces to urge Blair to abandon the Birt plan and bring Brown into the heart of the election campaign. Campbell describes as "total fiction" the account in Dennis Kavanagh and David Butler's book of the 2005 campaign of his telling Blair that "he should consider standing down" if he were not prepared "to work more closely with his Chancellor". We have only two weeks longer to wait for the "totally factual" account of those critical days in March 2005 in Campbell's diaries. But the sense of those discussions is clear, and even clearer now.

Gould and Campbell were worried that Blair would lose the election if he didn't bring Brown into the campaign. As the Birt papers confirm, Blair did not want to. But, two days after the campaign began, Blair's capitulation was made public. The terms of the surrender were unveiled, literally, by Blair and Brown at a joint photo opportunity on 7 April 2005.

They revealed two posters, one with giant mugshots of Blair and Michael Howard that said: "Who do you want to run the country?" And one with Brown and Oliver Letwin that said: "Who do you want to run the economy?" Hardly anyone noticed at the time - except Matthew d'Ancona in The Sunday Telegraph - but this was an explicit promise that Brown would remain as Chancellor. In retrospect, it was the moment that Brown secured the prize that he goes up to the platform to collect today.

That triumph is a tribute to Brown's sheer persistence. He has made it, not because he has any sort of relationship with Blair, but because Blair was forced to go and no one else had the right set of qualities to be Labour leader and prime minister.

By January 2005 it was clear that they did not get on well at a personal level at all. That was when Robert Peston, in his book written with the co-operation of Brown's right-hand person, Ed Balls, reported: "Brown now routinely says to Blair, 'There is nothing that you could ever say to me now that I could ever believe.'"

Of course there was not: that is not how politics works. It is about power, who has it and who can use it. Blair was only ever playing psychological games, and for a long time was better at it. But Brown learned, and finally prevailed.

There are two approaches to politics. Either can work, if you are good enough at them. There is clunking mechanical persistence. Or there is creative agility. The Blair-Brown story is one classic case study. The race for the Democratic nomination in the US is another. Barack Obama is the lightweight-uplift-Blair candidate; Hillary Clinton is the heavyweight-heartsink-Brown candidate. Nobody likes her that much but they know she's serious. She has carefully taken the electorally robust position on every issue, and she has avoided mistakes. In the spread-betting market she has steadily opened up a huge lead over Obama since mid-April, when he was the surprise darling of the early campaign.

In the same way, Brown has responded judiciously to the electoral pressures on him: in favour of academy schools and against the use of sofas in government. Yes, but surely Brown's talks with Sir Menzies Campbell and Lord Ashdown last week were evidence of a new lightweight, pluralist Brown? Not really. It was politics-by-numbers, following the template provided by more innovative politicians before him: Blair in 1997; Nicolas Sarkozy earlier this year.

This week, we move to the next, most explicit case study: Gordon Brown versus David Cameron. Brown sounded lighter, more cheerful and less relentless in his long Newsnight interview on Friday, and even gave every appearance of answering the questions and wanting to answer them. But he is still downbeat and duty bound, against the light, shade and sunshine of the leader of the opposition. They promise a struggle of interest and complexity matched only by that among the Blairites - and within the complex personality of Blair himself - between wanting Brown to win and thinking that Mr Lightweight Uplift has the edge.

Further listening: Hear the Baroness Morgan interview at bbc.co.uk/radio4/today/listenagain/tuesday.shtml

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