Watch out, Gordon, he's right behind you!

Mr Brown's anniversary in power tomorrow is a weary milestone, to be passed but not celebrated. According to a Guardian/ICM poll published today, David Cameron's Conservatives have a 20-point lead over Labour.

Of the situation, the best assessment I have heard from senior Cabinet figures is that Labour's underlying strengths are greater than they look now (they could hardly look worse) - and that the Conservative Party's weaknesses and fissures have yet to be fully dissected.

No one has so far risen to exclaim with conviction: "Gordon is a bloody good Prime Minister and we would be mad to even think about changing him."

On the Right of Labour, the mood is hopelessness tinged with self-pity. "How did we get stuck with Gordon when we knew he wasn't any good?" moans one backbencher. Many feel that they missed a chance in the wake of the Crewe byelection to set a deadline for Mr Brown's recovery - or dismissal.

Their great hopes - David Miliband, or the other hotly tipped non-entrant to this virtual leadership race, James Purnell - are unwilling to play assassin, even by proxy.

Just invert Murder on the Orient Express: everyone is relying on someone else to do it. The consensus, as one ambitious minister sighs, is: "Make the best of what we've got."

While everyone has been watching the Blairite redoubts for signs of open dissent, something else has been going on in the undergrowth. The figure emerging as hot to trot and waging a leadership campaign in all but name is the Schools Secretary, Ed Balls.

Mr Balls is clever, self-confident and utterly determined to succeed his old boss. As the PM's right-hand man from the nether depths of pre-1997 onwards (he was even in Granita as Gordon's sidesman on the night of the Blair-Brown deal), he inherits a network of links across to the parliamentary party and beyond.

Naturally, he cannot undermine the figure to whom he has long been first a political son and is now a ministerial mainstay and adviser. But he has every intention of being his replacement, when the time comes - and is increasingly adamant that he is his own man. So much so that the authorship of his "big ideas" such as the multi-faceted "children's plan" is hotly disputed among those who notice such things. Mr Balls claims credit - and has a Sovietsize mural in his department to make the point. Other allies of Mr Brown insist it was really the PM's longstanding idea, sketched out over his brooding years.

No one at senior levels of New Labour doubts that Mr Balls has been working party organisations and the trade unions assiduously with this goal in mind. One former chairman says wryly: "Wherever I went somewhere on a morale-boosting tour, they'd say, 'Oh, we've just had Ed Balls up to speak.'"

"The only campaign that is up and running is Ed's," says one former Downing Street aide. "He is the one who is focusing on the party selectorate and how to appeal to it. His message will be that ultramodernising New Labour has failed and we need to try something else."

A brief glance at his activities since becoming Schools Secretary last year tells its own tale. Mr Balls commenced by starting a row on admissions policy and in particular, faith schools. Labour activists adored this. In their eyes, middle-class parents were left free to take the best pickings from the state system unchallenged. Mr Balls duly set out to redress this - though it was avowedly not the preoccupation of reformers like Lord Adonis, who greeted the crusade with a blanket silence and got on with doing deals with the private sector on academies.

Up popped The Candidate again last week with a rallying cry: "Let's be clear. I don't support selection." This is a pure exercise in positioning, since not even Mr Cameron is suggesting extending grammars now outside areas which have them and there were no grounds to think Mr Balls did support selection. He can encourage grammars to take over other local schools, but not force them.

On some things, he has changed tack: converting from Brownite scepticism on expanding academies, albeit on terms more hedged and conditional than his Blairite predecessors or today's Tories. Heeding criticisms that New Labour has dallied too long in addressing failure, he has given underperforming schools deadlines to improve or close. That is quick thinking - even if belated.

His main flaw is a tin ear for what anxious middle-class parents think of the system: indeed, whereas many other politicians will naturally talk about the middle classes as a group they want to appeal to, I always have the sense Mr Balls is allergic to it and seeking ways to circumvent the question or answer in different terms.

One inconvenient roadblock he encountered this week was the CBI chief Richard Lambert tiring of Mr Balls's support for diplomas as an alternative to A-levels. I do not think this row has played itself out yet. Mr Balls was determined to unite vocational and academic qualifications in what he insists would be a high-quality diploma at 18. He cooptedthe engineering faculty at Cambridge to stake his claim that the new certificate would be accepted on grounds of academic excellence. Mr Lambert was his prime guarantor that business would like the idea. Beyond these limited quarters, the diploma has raised little spontaneous enthusiasm and now the CBI boss has peeled off, too.

So the Balls agenda is not, you might gather, to my taste. Yet at the kind of event last week where young politicians are keen to be seen and impress, he was the man who cut a confident, even swaggering figure. Mr Miliband and the other young modernisers are so worried that they will be seen to be jostling to power that they risk courting invisibility.

Self-belief counts for a lot when everyone else is waiting, watching and undeciding. Hurling himself on to a children's swing recently with hefty gusto for the photographers was a "Blair moment" gone wrong. What sticks in the mind is that he wasn't afraid to try.

A more assertive Mr Balls could yet wreak havoc in his own conflicted ranks. A contest with Mr Miliband (or another centre-Right figure) would become a contest for the soul of Labour along a Left-Right divide - with an influential Mr Brown backing his old friend.

The drawbacks of a figure so close to the PM angling to succeed him are there to be seen. "You've had Gordon: why on earth would you want Gordon-Lite?" scoffs one unimpressed minister. Also, Mr Balls is still an imperfect communicator who can be hesitant and a little lofty, though that can improve with practice.

He does have one big asset though: he wants the job and he is going after it, by hook and crook. That makes the other young pretenders look a bit weedy and unfocused on the prize. I should expect he has thought of that too.