There comes a moment in the life of all governments when voters start to feel that the governing party is more interested in its own survival than in the nation it is supposed to govern. This is such a moment. But David Miliband’s declaration of war on Gordon Brown is more brave than self-regarding, given that defeat at the next election seems inevitable and that he was most likely to be anointed in the aftermath.

His writing a call to arms in The Guardian yesterday, and omitting to mention the Prime Minister, does not need much decoding. It indicates Mr Miliband's desire to put paid to the “ditherer” label that so rankled after he refused to challenge Mr Brown last October.

There is no longer any doubt about his desire to lead. He has not (yet) struck a fatal blow. But if Mr Brown were to offer him the chancellorship in a September reshuffle, it is now difficult to see how he could honourably accept.

What may matter even more than who leads the party is what it stands for. Switching to a more media-friendly face will not end voters' frustration over Iraq. It will not sap the mounting desire for revenge on a government that has squandered too much of our money for too little benefit, leaving the cupboard bare now that we face the kind of economic downturn that Mr Brown claimed to have abolished.

It will not change the feeling that Labour has been utterly cynical about the people it is supposed to represent. We all know that politics is a con some of the time. It has begun to feel like politics is a con almost all of the time.

The hole that Labour is in goes deeper than the economy. So it is important to understand what “platform for change” Mr Miliband is proposing. His pitch is that a refreshed Labour Party must combine “government action and personal freedom”. But he is shy about saying where the balance should be struck. To be fair, he has been saying for two years that people want more control over their lives, and that Labour must devolve more power to people. He said it again yesterday - but without a whit of detail. The only policies that he mentioned sounded strangely like a manifesto for more government - windfall taxes on utilities, family-friendly employment laws, state-funded childcare, and “more protection from a downturn made in Wall Street”. (By which he emphatically did not mean letting taxpayers keep more of their own money - one of his aides laughed when I made that suggestion.)

He needs to elucidate. For this is the ground on which he wants to fight the other David. D. Miliband has been a shrewder analyst of D.Cameron than any of his Cabinet colleagues. From early on in the Cameron ascendancy he saw a serious threat. Now he has decided that Labour should define itself as optimistic, passionate about progressive ideals, and the Conservatives as pessimistic, bemoaning a “broken society” that doesn't exist. This is a more sophisticated argument than “Labour cares but Tories don't”, which was effectively neutralised by the Conservatives' “hands off the NHS” policy and its impressive work on social breakdown.

But the danger with making this the new fault line in British politics is that Mr Cameron got there first. In policy terms, it is the Conservatives who have so far seemed optimistic about the ability of people to make decisions for themselves, and Labour that has made devolving power to a few hospitals and headteachers look like an am-dram production, involving more histrionics and agonising than Racine. The irony is that where it has devolved most power - to Scotland and Wales - it has let nationalists hollow out its core vote.

Mr Cameron talks a better and better game about how state intervention can impoverish the relationships that are central to a good society. Mr Miliband also says that he wants more individual responsibility. Yet it is not at all clear how far he would go. Yesterday he said that “if people and business are to take responsibility, you need government to act as a catalyst”. That does not sound like a charter for freedom. In fact, it sounds implacably pessimistic about the ability of people to make the right decisions.

He says that “deregulation and lower spending” are incompatible with “social justice and better public services”. It is fashionable to refer to Mr Miliband as a Blairite, implying that he is more centrist than some of his rivals. They may be less intellectually able than he is, but his political DNA is not wildly different.

For all the handwringing, there still seems to be a gulf between Westminster and the country. Two Labour MPs told me yesterday that they saw no reason why Mr Miliband should not lead Labour into a 2010 election. But if there is one thing that would infuriate the public, it would be another prime minister being anointed after some masonic deal. Labour MPs will have to decide if they are prepared to risk their seats a year early to get a new leader.

None of this might matter so much if it were still possible for any Labour politician to do anger. Some (not all) can work themselves into a righteous lather against the Conservatives. But where is their anger about declining social mobility, family breakdown, rising knife crime? How can a Labour government have created a two-tier education system in which state pupils are discouraged from doing languages and sciences and from competing with private pupils who increasingly take different, and more respected exams?

Mr Miliband is one of the few politicians big enough to do humility, and clever enough to do rethink. There is no doubting his courage, and readiness for the fight. But he needs to understand just how deep a pit his party is in if he is going to have any chance of leading it out. Optimism won't be enough.

Times Recommends

• We have kit and leadership. Let’s go
• Behind the scenes at the biggest deal
• Conferences — what a waste of money