Last week Tony Blair argued that people’s weight is their own responsibility, but government has a crucial role to play in helping us to stay healthy.

It is easy to lampoon his speech on healthy living as a last gasp from a prime minister trying to avoid early retirement. But Blair was speaking about a significant social challenge that will require us to transform our diets and reorganise our health services around prevention rather than cure.

The trouble is, he is making the argument too late in his own political career for it to count. Blair has already ducked the “tough choices” that could have overhauled our health service, and already forfeited the personal credibility that could have made the public listen seriously to his message. For the next generation of political leaders, all that remains is to understand what lessons should be learnt.

When I left university in 1994 I wanted to see a centre-left revival in Britain. I also wondered how ideas were translated from ivory tower to Whitehall bunker. To find out, I joined a then recently established think tank called Demos.

Blair had just become leader of the Labour party, and it was beginning to look like a change of government was realistically possible. Nobody, though, imagined a political turnaround as dramatic as that of 1997.

The result was a political earthquake. Contempt for John Major’s Tories, Labour’s accelerated evolution under Blair, and a burst of post-cold war optimism combined to produce a redrawing of the political landscape.

Looking at this summer’s headlines, it is difficult to remember the energy this government generated in its early days. But it was heady stuff. Suddenly the think tank I had joined was part of the new Establishment, and my boss had joined the Downing Street policy unit. Not long after I got a call from David Blunkett’s office to discuss a job as a special adviser. At 24 I found myself walking the corridors of power, learning from the inside how the Whitehall policy machinery really worked.

Though Labour had swept all before it, the policy agenda was remarkably slim when it entered office. Blair faced a strange situation; getting used to an unprecedented level of power and control while trying to work out exactly what to do with it. Blair might not have realised that many of his best opportunities to change Britain would come early.

In 1999 I returned to Demos as its director. I have spent the past seven years watching and taking part in Labour’s efforts to be radical, and observing at close quarters the government’s slide towards disillusionment and political decline.

What is most staggering about Blair’s premiership is that it set out to restore public faith in politics and has ended up damaging it further. It is inevitable, in sceptical times, that an incumbent government will lose support — one estimate is that each term in power costs 3% of the vote. But the hallmarks of Blair’s government, and the way they have been represented to the public through the media, have clearly deepened many people’s sense of cynicism and disappointment at politics.

Undoubtedly, like all prime ministers, Blair has made key strategic mistakes that have undermined his premiership. Chief among these were the publication of the “dodgy dossier” on Iraq, his bizarre declaration that he would serve a full third term and no more as prime minister. Another may prove to be the decision to finance the 2005 election campaign through undeclared loans. But in fact the seeds of new Labour’s destruction were planted more deeply in the characteristics of its early success, which have blunted the government’s radicalism and limited Blair’s achievements. Ironically, several of them arise from the political dominance created by the 1997 rout.

The first characteristic is exceptionalism. Because Blair came to the leadership unexpectedly early, and proved the electoral saviour of the Labour party, it was too easy to believe that the laws of political gravity had really been suspended. This was reinforced by events such as the death of Diana, which Blair appeared to handle effortlessly while even the monarchy was rocked.

While Blair himself always warned against complacency, there was still a pervasive belief that the new Labour machine could somehow defy and define the terms of political engagement. Floating above this approach to power, like a filmy gauze, was the “third way” philosophy that even Blair no longer advocates.