Commentary: PM's weak spot is the growing fear that he is a personal liability

But it is another brick out of the crumbling wall of the Prime Minister's authority. This was not the long-awaited move of the "ultra" Blairites against Mr Brown, the historical reckoning, settling of old scores and drive for a different kind of progressive politics. It was, like one of those mutinies of the junior officer ranks in a troubled military campaign, a sign that dissatisfaction with the Field Marshal runs far deeper than just the phalanx of foes to the right of him in the party.

This is the aspect which alarms Mr Brown's allies more than anything else. His would-be assassins have loyal voting records: few would have been expected to figure as attention-seeking characters in the final New Labour drama. The strong female and southern representation was intended to signal that these are electoral target groups in which Mr Brown's weaknesses are most apparent.

Siobhan McDonagh put it best when she said that she simply did not want to have done nothing while the party freewheeled to disaster.

Today the fightback is under way. John Denham, a minister who has set himself up as speaking for the centre of the party, is a key player in No10's defence. In his Standard article today he throws down a challenge to the notion of leadership change: "Do they have a candidate? Not as far as any of us can see. Do they have a more convincing explanation of Britain's fundamental problems? Not that I have heard."

Of course, there is a candidate whose name everyone knows, including Mr Denham: the Foreign Secretary, David Miliband. These events complicate his position as crown prince.

Mr Miliband has always had a horror (some of his friends would say excessively so) of being involved in an assassination of Mr Brown, which he believes could lead to a bitter and long-term split in Labour's ranks.

One of the reasons this attack took even those closest to Mr Miliband by surprise was that the senior actors believed an assault need to be started from outside Cabinet.

Now the battle is on. Mr Denham and other loyal Brown voices today are shining a light on what they see as the central flaw in the "Ditch Gordon" case: that Mr Miliband is on-off about his own hopes, though his enthusiasm for Mr Brown yesterday was determinedly tepid: "I expect Gordon to lead us into the next election. I will support him in doing so."

The second point will be magnified in the week ahead and by Mr Brown at Labour's conference: what would an alternative leader change? There are few positions of principle or important policy that divide the party - though there are distinctions of direction and priority which would emerge in a real leadership contest.

But what motivated the gang of Amazonian knife-throwers at the weekend was not an argument over ideology, but a more basic one: they believe that Mr Brown is the problem and that Labour needs a chance to try something else. Here is the Prime Minister's Achilles heel; the growing impression that he is becoming a personal liability to his party. That's the key question set to dominate Labour conference and the rest of Mr Brown's time at the helm - long or short.