Ed Miliband and David Cameron have traded many jibes across the floor of the House of Commons, but one line still rings in Labour ears. It was hurled at Prime Minister's Questions on 1 December 2010. Miliband branded David Cameron and his colleagues "children of Thatcher". The Prime Minister came back quickly: "I'd rather be a child of Thatcher than a son of Brown." The first rule of name-calling is to apply a label that the target can't take as a compliment. On that score, Miliband's attack failed; Cameron's rejoinder did not.

Nearly a year since Miliband became Labour leader, the "son of Brown" charge remains potentially deadly, although not for the reasons usually cited. The publication of Alistair Darling's misery memoir of his time as chancellor has brought Westminster out in a rash of ugly reminiscence: the rows, the feuds, the plots. The morbid dysfunction in that political family is now a matter of settled historical record. Awkwardly for Labour, Ed Miliband was at the kitchen table when the crockery was flying. Worse, Ed Balls, the shadow chancellor, is accused of rummaging in the cupboards to keep the enraged patriarch supplied with missiles. Must their careers henceforth be scarred?

Mission defined

Not necessarily. Plainly it is a problem when a leader who wants to represent a new generation gets trapped in the footnotes of old-generation books. But the emphasis on personality clashes obscures a vital point. The Brown administration did not fail just because the prime minister got cross. Battles over policy, while clearly debilitating, do not have to destroy a government. The rows were mostly symptoms of a bigger systemic failure - Brown's arrival in Downing Street without a clearly defined agenda for government. He pestered his way to the crown and then seemed not to know how to wear it.

Tony Blair's regime was hardly short of rebellions and turf wars. Two of the many reasons it survived for a decade were that he was always on a clearly defined mission and always good at telling people what it was. Even when sections of his party hated him for it, he was in control of the agenda. Cameron understood this in opposition. His project to "decontaminate the Tory brand" was much derided by Labour and by some measures it failed. The Tories fluffed an open-goal election, scoring only a hung parliament. Now, Murdo Fraser, a candidate in the contest to lead the party in Scotland, wants to drop the Conservative name altogether, so tainted is the brand north of the border.

Yet Cameron's plan still worked at one level. People who did not spend all of their time thinking about politics - in other words, most people - still had a sense of what the leader of the opposition was trying to do.

What is Ed Miliband trying to do? One piece of the jigsaw is contained in Refounding Lab­our, a programme of party reform devised by the shadow Welsh secretary, Peter Hain. The idea is to revive Labour's grass-roots activist base. One proposal is to recruit an army of "registered supporters" - people sympathetic to the party but reluctant, at first, to become full-blown members. Hain also has his eye on the database of around three million affiliated trade-union members as a campaigning and recruitment resource.

The unions smell a plot to dilute their influence and some tussle over the issue is inevitable at the party's conference this autumn. The leadership is fairly relaxed about that. "It wouldn't be bad if there was a bit of a fuss," says one shadow cabinet member. "If it went through on the nod, no one would notice."

Getting noticed at all is one of the toughest parts of being out of power. Miliband, meanwhile, remains scornful of the profile-raising stunts that Cameron pulled, striking arresting poses, famously taking to the Arctic to highlight his (short-lived) interest in climate change.

But members of the shadow cabinet are increasingly worried about what has come to be referred to as the "no huskies rule". What, they ask, is the alternative? The Labour leader's position has been bolstered by assured handling of the phone-hacking scandal and the riots. But even close allies concede that momentum is a problem. "He has to keep sprinting just to proceed incrementally," says one.

Miliband's resistance to branding exercises flows in part from his determination to break with New Labour. One of the least attractive features of the Blair years was the habit of confusing presentation with action, dousing political fires with a fine spray of announcements. Miliband also explicitly rejects the technique, deployed by both Blair and Cameron, of seeking cheap definition by picking fights with his own MPs.

True Labour

There is something admirable in Miliband's quest to define himself without recourse to gimmicks and without alienating his party. He wants, say friends, to win power by effecting a more profound transformation in Labour than Blair or Brown managed - not compromising with the wild excesses of globalised capitalism, as New Labour did, but offering a substantial alternative. This aim is also noble but it is time-consuming. As one source close to the leader puts it, half approving, half frustrated: "Everything has to be a redefinition of social democracy for the 21st century."
In that respect, Miliband is a completely different proposition to Gordon Brown. He is obsessed with the big picture, while his predecessor was bogged down in pernickety manoeuvres. Their temperaments could also hardly be more different. But the longer Miliband takes in naming his agenda, the more people will speculate that he doesn't have one. Labour has already had one leader who defined himself against Tony Blair without adequately explaining what he was for. That leader also spoke passionately about social justice and ending poverty, but could not articulate how he would use the power of government to achieve those aims - other than by spending money. Now there is no money.

Forget the feuds and the temper tantrums. Knowing that you want to change the world, but not persuading the country that you know how - that is the danger that hangs over Ed Miliband. That is the mark of Brown.

Rafael Behr is chief political commentator of the New Statesman