John Rentoul: Miliband couldn't win. Better to wait and, should the worst happen, succeed Brown

Brown is too jealous of power to build up a rival Chancellor

The Blairite ultras, like most small political groups, divide into two factions. There are the realists and the fundamentalists. The realists have accepted for some time that Gordon Brown is going to be Prime Minister and have made their accommodations. The fundamentalists, on the other hand, continue to cast around desperately for someone - almost anyone - to take on Brown.

Frank Field, the former welfare minister, last week called on his party to "skip a generation" and choose David Miliband as leader instead of Brown. He is not alone. Miliband has been urged privately by many colleagues to change his mind and allow his name to go forward, presumably with a suitable show of reluctance, for the good of the country and the party.

Now, I may be an ultra - nowadays defined as anyone who thinks that Tony Blair has been on balance a good Prime Minister - but I am also a realist. Miliband is not going to run against Brown; not unless something extraordinary happens. Nor should he. Not because he would not be a better Prime Minister than Brown. The ultras think he probably would be. Indeed, we can name several other people who could be better, such as John Reid, Alan Johnson, John Hutton, Alan Milburn and Charles Clarke. And this is not to say that Brown will be a bad Prime Minister; simply that others might be better. But the party will not have them.

Miliband should not put himself forward because he would not gain the public support of the 45 MPs needed to put his name on the ballot paper. That is not because he is unpopular, but because he is young and untested; because the opinion polls do not suggest yet that he has the magic to revive Labour's ratings; and because Labour MPs have already committed themselves psychologically to Brown as unstoppable. To try to take on Brown and fail would only humiliate Miliband and set back the New Labour cause.

The sole candidate who stood a chance of making it to a ballot was Alan Johnson, because he could have added some MPs from the left to the handful of ultras. But his run faltered around the time of Blair's farewell party conference in September, and he pulled out in November, calculating, correctly, that he could still secure the deputy leadership. Last week, that mission came closer to fruition, when seven Blairite ministers - of the realist faction - went public with their endorsements.

Those realists, including James Purnell and Beverley Hughes, have the right idea. With Brown's succession hardened into fact, the Blairites have to focus not on fantasising about stopping it but on the battles that can be won: who will be Brown's deputy and who will be his chancellor?

For deputy leader, the choice is between Johnson and Hilary Benn, Secretary of State for International Development. Benn disqualified himself two months ago. "We need as deputy leader," he said, "someone who will ensure that the voice of the party and the unions get heard in the upper reaches of government." The idea that the trade unions should have a representative around the Cabinet table was one that Blair got rid of 13 years ago, and it should not come back.

For Chancellor of the Exchequer, the Blairites face a dilemma. That is the job that Miliband should get. Then we would get a green tax policy worthy of the name. But he will not get the job and we will not get the serious green economics. Brown is too jealous of his own power to build up a rival, and too cautious to take risks with an electorate that is only skin-deep green.

Then the question is whether Brown will appoint his long-standing adviser Ed Balls, currently a junior minister. Balls may be a Brownite ultra, but Blairites should applaud his promotion because it would show daring and self-confidence - qualities they do not associate with the present Chancellor. It looks as though Brown will flunk that test too and simply move Alistair Darling from the Department of Trade and Industry.

Hence Miliband spoke a truer truth than either he or his audience could have imagined when he made his Freudian slip on the BBC's Question Time 10 days ago. He predicted that, within a year, people would be saying: "Wouldn't it be great to have that Blair back because we can't stand that Gordon Brown?"

This was celebrated by practitioners of gotcha journalism, including those employed by the BBC, as a gaffe. No matter that his sense was plain - that whoever succeeds Blair will quickly become the target of Britain's ferocious anti-politics culture - the words could be construed as a slight against Brown.

What was interesting, though, was what happened next. Miliband did not back off. On Monday, he wrote an article for The Times calling on the party to be "bold Labour, not old Labour". On Wednesday, he gave a speech to US politicians in which he asserted the primacy of green imperatives over "treasury and finance ministers" because "climate change could cause a greater financial meltdown than the two world wars and the Great Depression put together". (He gave the speech by video link, not to cut his carbon air miles but because he had to stay in London to talk turkey about bird flu.)

The urgency with which he has seized on green politics is a standing rebuke to Brown. He was right on Question Time to imply that Brown would fail to embody "the change" that Labour needs. At the moment, Brown can still pose as the hope of something different. Because he does not go on Question Time or give many long interviews, we have not heard him express a view on gay adoption or road pricing or nuclear weapons. The moment he takes over, however, he will be required to express a view on everything. And on most subjects, he will personally identify himself with government policy and thereby, almost every time he says anything, he will disappoint everyone who had hoped for a change.

Miliband's green zeal, meanwhile, is also his personal insurance policy, ensuring that Brown keeps him as Labour's human shield against the growing environmental crisis. The way in which Miliband has wrested back the green advantage from Cameron in recent months guarantees his status at the top of politics for the next few years, come what may.

He will not challenge Brown this time, and he is right not to. But he is by some distance the leading candidate to be Labour's leader after next, should Brown, as the Blairite ultras fear, lead the party to defeat.