By his friends shall you know him. Gordon Brown may not have been aware of the precise content of the e-mails sent from his head of strategy, Damian McBride, to a Labour blogger, Derek Draper. But he appointed the author of the smears as his chief media adviser, first at the Treasury and then at No 10. He encouraged the aggressive culture that allowed such nastiness to breed at the highest level in his Government. He honed the politics of negativity by concentrating on dividing lines with his opponents, whether Labour or Conservative.

The Prime Minister cannot just shrug this off as the act of a rogue agent. Spin doctors are like the daemons in Philip Pullman's His Dark Materials - not only do they sit loyally on their master's shoulder, they also reveal the character of the one they serve.

Mr Brown is in many ways a high-minded politician. He genuinely wants to change the world, with a global new deal and a strategy for combating poverty. The recession has played to his intellectual and ideological strengths. But as well as waving his moral compass, he has always wielded a dagger and surrounded himself with thugs who are not afraid to use it on his behalf. Ministers speak of there being a split between “good Gordon” and “bad Gordon” and when the Prime Minister feels vulnerable it is Mr Hyde who triumphs over Dr Jekyll.

“He's got a nasty, aggressive side and a kinder more intellectual side,” says one senior party figure. “Perhaps over the years the balance between the two has gone wrong.”

It's not just Mr McBride who is the problem. There is a laddish and bullying atmosphere to the cabal of advisers and MPs surrounding Mr Brown. Small talk revolves around football. Briefings take place in pubs and karaoke bars. The alleged coup against Tony Blair was planned over balti and beers. It is not surprising that Mr McBride begins his e-mail with the word “Gents” - the underlying misogyny of the rumours he was trying to spread is one of the most shocking aspects of the whole thing. “Gordon is from Mars and more than half the voters are from Venus,” one female minister says.

There have for years been political assassinations, knee-cappings and punishment beatings for those who cross the godfather. I remember, during my first week as a lobby correspondent in 1996, hearing Charlie Whelan, Mr McBride's predecessor, telling another journalist that he would have “no stories for three weeks” because he had written something unhelpful to Mr Brown. I assumed he was joking. I quickly realised he was not.

Poison is nothing new in politics, of course. In House of Cards Francis Urquhart dripped it with the catchphrase: “You might think that - I couldn't possibly comment.” Bernard Ingham and Alastair Campbell could both be venomous when they wished. But there is a powerful tribalism to the Brownite inner circle. Outsiders are friend or foe. And enemies must be destroyed.

The roll call of victims is long - and until now there have been more Labour names on it than Tory ones. When Mr Brown was at the Treasury, potential leadership rivals were taken out one by one: John Reid, Alan Milburn, David Blunkett, Charles Clarke all saw their ambitions sunk by anonymous briefings. Even Mr Blair was forced to leave early in the end.

When Mr Brown looked vulnerable last year, the young pretender David Miliband was accused by Downing Street sources of being disloyal and immature. Rumours have been spread that James Purnell is gay - something that is totally untrue. Alistair Darling has been reshuffled countless times by unnamed advisers. When the going gets tough, the Brownites even turn on each other. Douglas Alexander was hung out to dry over the election that never was. Stephen Carter, brought in to shake up No 10, was quickly seen off by Mr McBride, who briefed journalists that he was politically naive.

The Prime Minister is never personally involved in the dirty tricks, of course - as with all covert operations, there is plausible deniability. But Mr Brown cannot claim to be blameless when he continues to surround himself by attack dogs who bite on his behalf. Indeed, the Prime Minister is himself a naturally aggressive player on the political field. He has long argued that Labour should use the class card against David Cameron - something always resisted by Mr Blair. Some Labour insiders think “bad Gordon” was fattened up by his sense that his leadership ambitions had been thwarted for so long. Certainly, the “war years” in the Treasury bunker strengthened the Brownites' sense that it was them against the world.

A growing number of ministers are convinced that Mr McBride and Mr Whelan have in recent months been promoting the leadership ambitions of their old friend Ed Balls. They detect familiar tactics being deployed on behalf of the Schools Secretary, with a slow drip of negative stories about potential rivals such as Ed Miliband, Harriet Harman and Alan Johnson. This may be political paranoia but it says something about the state of the Labour Party that such speculation has taken hold.

Mr Brown may have ordered a clean-up of the code of conduct but Mr McBride was not a lone assassin. His e-mails show that the Brown machine is still driven more by anger than by hope, keener to destroy opponents than to set out its own plans. If the Prime Minister could present a compelling case for people to vote Labour at the next election then his strategists would have less need to resort to dirty tricks. But it appears he cannot. Political parties turn to negative campaigning when they have nothing positive to say.

In 1997, the Tories portrayed Mr Blair with demon eyes as they had run out of steam to govern. Smear campaigns may fill a vacuum but in the end - as Mr Brown will almost certainly discover - they suck those who use them into an inescapable and utterly destructive black hole.

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