I t is as though we had let a dirty bomb into 10 Downing Street. The language used to describe the operation of Damian McBride and other Gordon Brown acolytes is the language of terror. Some of the words that tumbled yesterday from the mouths of Labour former victims of the No 10 briefers were more suited to people subject to chemical attack than to mere political hostility.

“The most lethal attack machine in the history of British politics,” said one; “they have polluted the core of British politics for years.” “Licensed to kill, by Gordon,” said another.

“Red-on-red action,” muttered a third. “These e-mails are the minutest tip of the iceberg. For years and years and years it has not been the Labour Party’s political enemies who have been on the receiving end, it’s been people in the Labour Party.”

Nervous ministers have been warning of “increased activity” from Mr McBride in recent weeks, an echo of the language the police and MI5 use when monitoring terrorists about to plan an attack. The language of dirty bombs is appropriate, because the sort of smear operation specialised in by Mr McBride – of which the weekend’s e-mails are a stark example – is about poisoning the political atmosphere. Like a corrupted version of what was itself a corrupt system, Richard Nixon’s Plumbers, these guys try not to stem leaks, but to drip poison into the system.

The keys to the system are held by journalists. It is only through the collusion of journalists that underhand and anonymous attacks on political colleagues can have any effect.

The media are all chorusing now: we knew, we called him McNasty and McPoison, we had nothing to do with him, he sent us foul messages, we didn’t like him. But the point is, we did know. We may not have known the detail of the nasty smears about senior Conservatives that Mr McBride was dreaming up, but we knew about the smears against his own side. We knew what he was up to, and we knew that he was being paid about £60,000 a year of public money to do it – and we did nothing to stop it.

Mr McBride used the system of anonymous briefings under which political journalism operates to spread dirt about Labour opponents of Mr Brown. Should journalists still be under a duty to protect their sources when those sources are abusing public money, or should we have been bolder in exposing it? Mr McBride did not poison the well on his own. There has long been a “dirty tricks” cabal around Mr Brown that any Westminster journalist or minister could name – Ian Austin, Tom Watson, Ed Balls, Mr McBride and, formerly, Charlie Whelan, who is now political officer of the Unite super-union (and working hard to place favoured candidates in winnable seats for the next election).

The poisoning was at its worst in the run-up to the leadership noncontest two years ago. Yesterday I spoke to somebody who balked at challenging Mr Brown then, because he couldn’t face the poisoners. “It’s the reason why Gordon came to office untested,” he said. “When I considered challenging him for the leadership, people warned me it would be a very unpleasant campaign; and it would have been an unpleasant campaign because Gordon’s people would have run it in an extremely vicious way.”

As he spoke, I remembered being told at the time by a number of journalists that one potential candidate was having a mental breakdown, and there was some embarrassing story involving him and a woman doing the rounds. The tales seemed obviously to have been invented by Mr Brown’s muck-spinners. In place of ideas, smears: that contest should have been conducted by open debate, not whispered poison. But Mr Brown was afraid of the debate.

And look at the result: in place of policies for the future of Britain, we have Mr Brown’s chief strategist inventing stupid smear stories about the Tories: what a devastating metaphor for a political and moral vacuum. Frank Field, MP, blogged the following anguished comments yesterday: “Week after week MPs have been turning up but with almost no serious work to do. There is the odd Bill to be sure. But there is no legislative programme to speak of. Even the debates that are put on to fill in time are ones that deny MPs a vote. The whole exercise is vacuous.”

Getting rid of Mr McBride is not going to resolve that, nor will it end the culture of terror. Senior Labour figures are aware that the techniques used to stifle dissent against Mr Brown in the past are already being used to manipulate the succession in favour of Mr Balls. “They are going to do it again with the succession to Gordon,” said one former Cabinet minister yesterday, “because it’s worked for them so far. They are having a good go at Harriet [Harman]. They are beginning to train their sights on James [Purnell].”

I recently had a conversation with one potential contender for the Labour leadership in the future who said he was not sure he could face running, because “they” would come after him. “They got David [Miliband], they’re getting Harriet [Harman], I don’t want to be next.”

In anonymous briefings to newspapers, Mr Miliband has been branded immature and self-serving, Harman pushy and stupid. They will pick them off one by one.

Not even the Prime Minister could stop this now, even if he wanted to. “This was not just an error of judgment, these e-mails,” another former Cabinet minister put it. “It’s a total error of character. These changes to the rules about special advisers are completely and totally irrelevant. It’s not about rules, it’s about the moral compass of those involved.”

And that includes some in the media. On the one hand, as one victim of hostile Downing Street briefings puts it: “If somebody at No 10 is saying that Harriet Harman is having a mental breakdown, journalists are justified in running it.” On the other hand, as another victim of Mr McBride’s sees it: “It takes two to tango – you need McBride and you need the flopsy-bunny journalists who will just take the line.” I think they should be clearer about where the line is coming from, and why.

Do you see what I did just then? I placed in your head the creeping falsehood that perhaps Ms Harman might be having a mental breakdown; because if someone has suggested that No 10 might be saying it, then perhaps No 10 is saying it, and perhaps it is saying it because it might be true?

And you will remember, long after you forget everything else in this article, that someone once told you that Harriet Harman was mentally ill. And that is precisely how the poisoners operate.

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