John Rentoul: Summer's over. Let the plotting begin

Gordon Brown's tenuous hold on power is all returning MPs can talk about, and his fate lies in the hands of two men

I'll shut up about the Labour leadership in a moment, but the trouble is that everything comes back to it. It would be worth writing about education policy, but Ed Balls was the cause of the problem last week. And Gordon Brown is the cause of Ed Balls. On Monday ministers opened new and refurbished schools, including 67 new academies, in a start-of-term blitz that could have painted Labour as the party of excellence and choice. Instead Balls used it as a platform to attack "Tory cuts". It would be worth writing about the economy, tax and spend, health, defence, benefit dependency and climate change, but in each case the Prime Minister makes it harder for Labour to match the half-formed policies of the Conservatives.

The question of whether Gordon Brown can survive until April, when he is expected to see the Queen to ask for the dissolution, is not going to go away. And it is becoming increasingly clear that the answer depends on two people: his Chancellor and his Foreign Secretary.

If that observation had been made at the start of the summer, the common response would have been that a change of leadership was not, therefore, going to happen. Alistair Darling has long been regarded as one of Brown's most loyal allies. The ties are personal as well as political: he and his wife Margaret have been the Browns' babysitters. The common view of David Miliband, meanwhile, is that he showed a lack of courage in June by failing to join his friend and fellow Blairite James Purnell in resigning from the Cabinet and calling on Brown to go. Both of those assumptions are flawed.

That does not mean that Alan Johnson is certain to replace Brown before the election; simply that it is more likely than many think.

As the political season starts up, so does the scheming. They are returning to Westminster, and there is only one subject of conversation. Labour's dire position in the polls has not changed; "why can't they see that any change is better than going down to certain heavy defeat?" is a surprisingly common, if minority, view.

Two things happened last week. When the verdicts in the plane bomb plot were announced, the Home Secretary appeared on television. I cannot remember a thing that he said, but, at a moment when the nation was paying unusual attention, he spoke in clear, simple language and sounded both perfectly reasonable and completely prime ministerial. The other was a report that an MP would stand on a "Gordon must go" platform for Labour's Parliamentary Committee, which is elected next month. This group is like a shop stewards' committee for Labour MPs, and meets the Prime Minister every week to keep him in touch with backbench opinion. On its own, this device is not going to push Brown out, but it allows Labour MPs to express their view in a secret ballot, so it is likely to attract the support of many more than the 23 that have publicly called for Brown's head (excluding members of the hard left Campaign Group). One MP told me: "There are more Blairites in the PLP [Parliamentary Labour Party] than there were when Tony was leader."

Everyone knows, however, that Brown cannot be forced out unless the Cabinet turns against him. Most ministers say that the mood is fatalist and they doubt whether Alan Johnson really wants the top job. There are only two people at the Cabinet table that really want to be prime minister, one told me: Gordon Brown and Ed Balls. I am not so sure. I think that Johnson is ready and willing to serve his party and his country, but cannot be the beneficiary of his own coup. Perhaps he should do more to signal his availability, but his Cabinet colleagues have to do the dirty work.

So who could go to Brown and suggest that he should stand down? Not Peter Mandelson. Of course, Mandelson was wrongly blamed by Brown for knifing him in Tony Blair's favour in 1994, so Mandelson ought to redeem the unfairness by knifing him for real now. But he won't. The history of his relationship with Brown – "I was the third person in the marriage; I was the casualty" – would make it too painful.

The task must fall, therefore, to the other two important members of the Cabinet. I doubt that Brown could survive the resignation of both his Chancellor and his Foreign Secretary, and so he would probably stand down before any such threat were made public. That is why it is worth looking again at the conventional wisdom that neither Darling nor Miliband would do such a thing.

The parallel between Darling and Geoffrey Howe is striking. Howe had been humiliated by being moved from the Foreign Office. Darling had to endure Brown's semi-public attempt to give his job to Ed Balls. There is of course one difference, which is that Darling saw Brown off: he refused to be moved from the Treasury; and he won the argument against Brown's attempt to fight the election on "Tory cuts versus Labour investment". But the fact that Darling is stronger than Howe is hardly reassuring to Brown. Howe was never Thatcher's babysitter, but no one thought he would do it either.

Then there is David Miliband. Some anti-Brown MPs were vitriolic about his failure to join Purnell in resigning in June, but his supporters argue that this was the wrong rebellion at the wrong time. If they had both resigned, they could have been dismissed as embittered Blairites and Brown would have survived – not least because Labour MPs did not want to risk an early election. Now the election is almost on us. All Miliband has to calculate is whether he wants to inherit a train wreck or to try for the leadership after Johnson has saved something from the cataclysm.

Brown has one last throw of the dice at party conference – I am told that everything is being thrown at his speech. But if, or perhaps when, that fails to lift Labour in the polls, the Prime Minister has nothing left to fall back on. Only Darling and Miliband. His fate is in their hands.

John Rentoul's blog is at independent.co.uk/jrentoul