Debt, and what to do with it, has long been the essence of practical politics. Debts to the party which promotes, to the voters who elect, to the ideology which gives the framework of thought, to the associates who advance the career and teach the tricks, to the families who forbear to press their claims for time, to the officials who forbear to press their claim on the politician's latest initiative - all of these debts can be called in at any time, used to embarrass or discipline. Peter Mandelson's debt to Geoffrey Robinson was unusual between politicians because it was in coin. There are heavier debts.

Tony Blair came into government with two of the heaviest. One was to Peter Mandelson, for his assistance both in creating new Labour and in creating new Blair, the leader. It was a debt that, though heavy, was nevertheless transparent. Mandelson had chosen to devote his talents to Blair's advance and, though the devotion was beyond what most in his position were capable of, and though he was prolonged in his non-cabinet post as minister without portfolio longer than he wished in order to be available on the same terms to Blair as before, still he signed up for it. He could be let go, in extremis, without guilt. The slaying was epic in the popular use of the term, but not tragic in the strict sense.

The other debt is of a different order. Gordon Brown was Blair's closest political friend and mentor, who had not only taught the slightly younger man and more recent MP the ropes, but also gave him - as Blair would gladly testify - an intellectual structure with which he could grasp the challenge of party reform. Brown had done much of the work that has become known as new Labour while Neil Kinnock and John Smith were leaders of the party, in close association with Mandelson, among others. Blair had a powerful mind and good judgement and, being less of a Labour family man, was less inhibited than Brown. But he was, throughout the eighties and into the nineties, in Brown's debt. When he went for the leadership, he was transgressing what he himself had felt to be the natural order of things - that Brown should be leader.

This explains the enormity of the concession he made - essentially giving away a vast area of government to Brown. Paul Routledge, in his biography of Brown, says that the deal they reached was that, if power was gained, Brown would be Chancellor with "full charge of economic policy and a powerful influence across the range of social policy". This was far more than the understanding that one baron would be more powerful than the others (the Chancellor, in British cabinets, tends to play that role ex officio in any case). It was the recognition of a separate kingdom.

Hence the feuding between No 10 and No 11 was literally built into new Labour's system. Payment of the debt has meant that the borders of No 11 have been pretty much inviolable; or that, if one scored a hit over the other, the other had to be allowed revenge. When Harriet Harman, the social security secretary and a Brownite, was fired, her deputy Frank Field, a Blairite, had to go, too. When the Blairite Mandelson went, so did the Brownite Geoffrey Robinson, the paymaster-general (and the proprietor of the New Statesman). The extraordinarily close group that Brown gathers about himself - Ed Balls, his economic adviser, Ed Miliband, his speechwriter, and Charlie Whelan, his press agent - make policy not only for the Treasury, but for a large part of the government.

Mandelson's original sin to the Chancellor's camp was in siding with Blair rather than Brown after Smith's death. His continuing sin was that he bent his best efforts to ensuring that Blair would be an historic Prime Minister. Often, he felt this was best done by putting Brown in his place; he would unattributably brief against him, even while conceding his political force and regretting a breach for which he blames Brown's vanity and intolerance.

Brown's instrument among the media has been Charlie Whelan, a former press officer of the Amalgamated Engineering and Electrical Union and on the left wing of that once totally ideologically riven body. Whelan is the best indication that the Chancellor's court is a separate entity from the Prime Minister's; he sees his job as analogous to that of Alastair Campbell (the Prime Minister's press secretary) in the sense that his briefings are devoted to promoting the Chancellor's agenda, image and initiatives as if he were at the head of the government with his own policies.

Blair had tried to get Whelan fired immediately after the election; it was a major part of the first conversation the two men had. But Brown would not do it. Now, the pressures against Whelan mount once more. It is generally assumed that he told the Mirror journalist Paul Routledge - who has just completed a biography of Mandelson - that Robinson had lent Mandelson £373,000 in opposition to assist him to buy a house in Notting Hill Gate. The story is in the first chapter of Routledge's book, which begins serialisation this month; the Guardian, which broke the story before Christmas, says it independently verified the Routledge account.

Whelan is a natural suspect; he closely advised Robinson while the paymaster-general was himself under increasing pressure from the media because of his business activities, and he has been close to Routledge for two decades. But, in one way, it would be surprising for one so skilled in the close-quarter, profane, butt-headed grappling that constitutes much of modern press relations. For in exposing Mandelson, he would open himself to sacking without a weapon to defend himself. As long as the loan remained secret, Whelan could put it about that, if he went, he would take Mandelson with him. Now, Mandelson has gone - and left him without his nuclear weapon.

Routledge had relied on Whelan's information when the former was a labour correspondent for the Times. The engineering union, whose left wing Whelan served, was a defining body for labour correspondents of the time; more than any other body, it forced the reporters to declare themselves politically. The executive committee members of the right drank at one pub, those of the left at another; reporters who sought out one group for information could not cross no-man's land to the other in the expectation of a welcome. In large measure they had to take one side or the other - a practice that operated, though in less well-defined form, throughout the union movement up to the mid 1980s.

Routledge dominated the labour correspondents' group in the late 1970s and early 1980s, from an uncompromisingly left position. His political and moral touchstone was the left wing of the National Union of Mineworkers, and especially Arthur Scargill, a fellow Yorkshireman. Both men's motto was: "What's mine is mine, what's yours is negotiable", a Bolshevik slogan that served both well for a long time, though they have not spoken since Routledge wrote Scargill's biography.

Thus both Whelan and Routledge should be - and consider themselves as being - very old Labour indeed, implacable enemies of the rocket salads and guacamole on which Blairites are supposed to be constantly nibbling in minimalist Islington restaurants. It says much for the catholicity of Brown's politics that he manages to attract to himself the likes of Whelan and Routledge, while actually being the main strategist of Labour's shift into the centre.

Yet both men have adopted, and perform to perfection, a practice that is very new Labour: total focus on the message, and a steady refusal to go off it. That which Mandelson is deemed to have made his trademark is their meat and drink.

Whelan grasped, more joyously than any other of the spin-doctors, that reporters in the hyper-competitive London media are desperate for story-lines and will not always check carefully that the line is accurate. He understood and brilliantly exploited the British tradition of "off the record" briefing, which protects briefer and reporter alike. The briefer can, in the service of a master or a cause, deny briefing and the reporter can refuse to divulge his or her source. This practice, in the hands of the new Labour spinners, has become more than the dubious, self-serving tactic it always was; it is a large part of the weaponry of internal feuding, and a very large part of the ammunition used by No 10 and No 11.

Equally, Routledge has grasped that the revelation of some facet of personality is now the most important commodity in political journalism. The decline of ideological and political combat within the leadership, and the constant extension of the media's belief that private life is relevant to the conduct of public life, puts a very high premium on the exposure of a sexual or financial scandal. It can turn a pot-boiler biography into a "sensation" - the word used on the paperback of Routledge's Brown biography, quoted from a comment about the book on Newsnight.

Much of this is quintessentially new Labour - or at least, the world which new Labour claimed to understand and to be able to manipulate.

Whelan and Routledge, with an old Labour agenda but with new Labour tactics, may well have been Mandelson's nemesis - and operated under the protection of a kingdom whose space was inviolate. Tony Blair gave it that status, in payment of a debt. The question for the new year is this: is the Prime Minister prepared to go on paying the debt? Or will he demand a renegotiation of the terms, to remove Whelan, whose power, many think, has been shown to be very great indeed?