Why did Andrew Mitchell have to resign [2] from his post as Conservative chief whip? The explanation has many layers. The facts of the original pomp-and-profanity episode remain obscure. Mitchell disputes the police record. Politicians sometimes lie; so do police officers. On the evidence alone that should make it a stalemate. What appears to have done for Mitchell is the feeling in the parliamentary Conservative party, especially among the 2010 intake of MPs, that his credibility and authority were shot.

It was never going to be easy for the chief whip to impose discipline when his own lack of self-control had been thoroughly advertised for the best part of a month. But why did the original allegations [3] matter so much in the first place? The toxic element was the suggestion of arrogant snobbery, which was only noteworthy largely because it risked reinforcing problems with David Cameron’s own image – aloof, unacquainted with hard graft, surrounded by a gilded elite.

One of the peculiar features of the whole Mitchell saga is that the only reason it was a “news” story at all was contained in tangential relevance to the wider problem with Cameron’s leadership. (Few people outside Westminster know or care who a chief whip is and what he does). Yet Cameron appears to have been the very last person to think that Mitchell’s outburst was significant. That is why Tory MPs are so cross with their leader. He neither grasped the emblematic power of the incident, nor found a way to contain the story once it was running. He – or rather the Number 10 operation he runs - showed faulty initial judgement and followed it up with ineffective political technique.

Even that doesn’t get to the essence of why this particular resignation is revealing. Most resignations do little lasting damage to a government and it is too early to say if this one will be any different. I think it does matter in one crucial respect: To recap - Mitchell didn’t resign over something momentous he had done or because the Prime Minister lost confidence in him as a result of things he was alleged to have done. He resigned because the stench of brand decay hung about him. It is the first instance I can think of where someone was sacked not because of something they said, but because of something representing just the kind of thing they might be expected to say.

The implications of this for the Conservatives are pretty serious. The same phenomenon could be observed on Friday afternoon when it emerged that George Osborne sat in a first class seat on a train without the ticket to go with it. This became a media event not because of evidence that Osborne intended cynically to evade his fare or kicked up a fuss when challenged. No such evidence exists. It became a story because sitting in first class with a standard ticket and pointedly refusing to move for fear of proximity to the great unwashed is, in the public imagination, just the sort of thing Osborne might be expected to do.

This is a step beyond ordinary communications problems. It signals the arrival of a new phase in the cycle of political decay. This is the point where the government struggles to get its message out because the official line cannot compete with negative stories that reinforce a pre-established hostile narrative. Anything that appears to support the worst interpretation of what it means to be a "typical Tory" in the Cameron-Osborne mould is newsworthy – pretty much regardless of whether it actually happened. And these were supposed to be the people to “decontaminate” the brand. No wonder Tory MPs are worried.