John Rentoul: His importance to the PM was vast. He was much more than a press chief

Alastair Campbell was in at the start of New Labour, even before the brand name had been devised. As political editor of the Daily Mirror in 1989 he was a regular visitor to Tony Blair's office in the House of Commons. Blair, recently elected to the Shadow Cabinet, had just been appointed by Campbell's friend Neil Kinnock to be the Labour Party's spokesman on employment.

He advised Blair on how to distance the party from the trade unions. After Kinnock's resignation in 1992, he became a member of the inner core of "modernisers" as they pressed their strategy for a "new" party on a reluctant John Smith.

When Blair became leader nine years ago, Campbell was at his side as the strategy was pressed into action. In tandem with Peter Mandelson, who was already an MP but had higher ambitions and chafed against his backroom role, he oversaw the rebranding of the party, with a new name and a new constitution.

The essential premise of New Labour was that the party had to be utterly ruthless in making the compromises necessary - principally in winning over a Conservative-inclined press - to secure power and implement whatever moderate social democratic policies its leaders deemed possible.

Campbell likes to present himself as a Labour Party person through and through, and certainly had some policy disagreements with Blair such as over his passionate commitment to comprehensive education. But much of New Labour's macho aggression in defining the politics of the possible derives from Campbell rather than Blair.

The Campbell-Blair campaign to persuade the right-wing press, still pining for Margaret Thatcher, that Blair was the man to lead them out of the dreary compromises of the Major years was one of the most stunning coups in modern British politics.

By the time Labour was elected, Campbell was already, therefore, much more than a press secretary. He and Jonathan Powell were the twin lieutenants of a radical centralisation of the government machine.

Campbell's importance to Blair as an adviser may not have been unprecedented, but his position of executive authority certainly was. No previous special adviser, and certainly no press secretary, would have demanded an explanation of a cabinet minister, as he did of Harriet Harman in the very early days.

So when Campbell protests, as he did yesterday: "I have no power independent of the elected Prime Minister", it is a statement worthy of being set as a question in a modern constitutional theory exam. It is true that his authority derived wholly from the Prime Minister, and that the Prime Minister always supported him. But you cannot sit on the "War Cabinet", or in all the most important meetings in the Prime Minister's office - of which the Hutton inquiry documents paint such a full picture - without exercising a considerable degree of influence in your own right.

The real test of Campbell's influence, of course, will be the difference his departure makes. Like Mandelson, he acted as a personal lightning conductor for Blair, diverting some of the resentments that would otherwise attach to his boss. But he could only do that by drawing attention to himself and committing the elementary error for a press secretary of "becoming the story".

This set up a paradox. After the election, Campbell tried to withdraw from frontline duty in the hand-to-hand struggle with Westminster journalists, assuming the title "director of communications and strategy", which was more honest about his role. At the same time, Blair committed himself to holding monthly news conferences with the media pack, so that they could have direct, unspun access. That did not do much to reduce the perception of spin, because most people understood by now that the Prime Minister was quite capable of spinning for himself - indeed, he was brilliant at it.

Campbell's departure means the Prime Minister will be more exposed, but he will adapt, seamlessly, just as he did to Mandelson's departure, which was also regarded as a loss that was bound to weaken him. But Campbell's retirement will have more of an effect on the political character of Tony Blair's Government, because he is the last of the architects of New Labour to leave the Blair circle.

His successor, David Hill, will not be filling the Campbell-shaped hole left at the heart of Britain's informal Blairite constitution - he is a press officer pure and simple and he will be the more effective for it.

That means that we may genuinely see a Blair Mark 2 as the Prime Minister turns to new sources of advice. The danger for New Labour will be if Blair draws solely on the advice of civil servants.

Some of Campbell's critics in the Labour Party may eventually find themselves wishing for the return of an explicitly political bruiser to the Prime Minister's side.