After 13 years in government we needed a proper post-mortem on why we lost, what went wrong and where we go from here. I nominated Diane Abbott because I wanted that debate to have as many voices as possible. Three months on, we have reached decision time. The question is which of the candidates can forge a credible and inspiring new project for the left.

For me, that question has been answered emphatically. It is David Miliband. He offers change in our party, understanding that Labour must become a movement again. Barack Obama was the first to grasp this in the Democratic Party, mobilising his volunteer force to help victims of the Midwest floods during his own campaign. David gets this, too. Already he has trained 1,000 community organisers as part of his campaign. In time, they will help communities speak with one voice about the things that matter to them.

Political parties can no longer be reduced to tools of mass communication; they must become forces for good in people's everyday lives. This is one step towards revitalising our party. Rediscovering our faith in party democracy is another. Significantly, David has proposed a democratically elected party chair. Members will have their own representative, speaking for them in the media and around the shadow cabinet. David offers a vision of people enjoying politics again, feeling proud to be in the Labour Party.

Alongside a change in party organisation, David offers the hope of a genuinely new political project. This means more than a shopping list of promises to different interest groups. Such a politics can appeal, but never stands the test of time. Instead, David promises a new direction. It was set out brilliantly in his Keir Hardie Lecture last month when he said that "New Labour was too hands-on with the state and too hands-off with the market".

The citizenship thing

Often when we were too hands-on with the state it meant that civil liberties were eroded. And the problem went deeper still. The state can come between people when piles of paperwork stop people volunteering, deny children the chance to go on school trips, or prevent mothers from looking after one another's children. When we try to run society from Whitehall, we show too little trust and respect for people as human beings in their own right. We end up replacing, rather than reinforcing a sense of community.

That we were too hands-off with the market is more than a comment on the credit crunch. It is to argue that the kind of economy we have and the type of society we live in cannot be separated. That was true when children were exploited in the factories of the Industrial Revolution and society chose to set limits on how people made money. It was true when women went to work during the war and rewrote their place in British life. It was true when the Tories wrote off millions of people during the recessions of the 1980s and 1990s.

The same is true today in a country where executives have the power to award themselves outrageous bonuses, where loan sharks exploit other people's poverty, where companies target advertising at children, where parents are made strangers from their children by the longest working hours in Europe, and where clone high streets are draining local identity. David offers change because he understands that a new economic model doesn't just mean more regulation of the banks, it means a market economy built on the values of mutuality, reciprocity and local decision-making. He gets that people should be able to make decisions together as citizens, not just be treated as consumers.

For this vision alone, I would support David. But there is one more vital thing that he will change: our habit of retreating in a comfort zone in opposition -- and staying there while the Tories do great damage to our country's social fabric. The people who depend on us cannot afford us to do this again. They need us to hold the government to account and to provide a credible and exciting alternative. In David Miliband we have one. I, for one, will be voting for him.