There comes a time in almost every Prime Minister's career when the thoughts of his colleagues turn to his, or her, successor. In the case of Tony Blair there was the problem that he was chained to his self-elected successor from the moment that he took office. But with Mrs Thatcher the end came very quickly at the end of her premiership, and was especially precipated by Geoffrey Howe's resignation speech in which he spoke of cricket bats being broken by the team captain, and criticised the nightmare image of Europe which he said that Thatcher had painted (entirely accurately as it turned out). John Major faced opposition from the beginning, much as Cameron has done. In an unguarded moment he allowed himself to be recorded sharing his opinion of his ministerial team...

Just think it through from my perspective. You are the prime minister, with a majority of 18... where do you think most of the poison is coming from? From the dispossessed and the never-possessed. Do we want three more of the bastards out there?

A continuing stream of scandals undermined what little authority he had. In 1995 he also faced open opposition and was challenged by John Redwood. The Sun newspaper saw which way the wind was blowing and switched support to Redwood using the headline - Redwood v Deadwood. He lost the 1997 election of course, and immediately resigned. But it was the open opposition of colleagues in the parliamentary party and even in the Cabinet which had finished him off years before.

It doesn't take much. At first there are the unspoken thoughts, not shared with anyone, that the Prime Minister might be a liability rather than an asset. There there are the whispered conversations wondering where the Prime Minister is taking the Party and the country. Then there is the stage of open resistance to various policies. And finally there are the calls for a challenge to the Prime Minister. At that point, unless the Prime Minister in question is able to perform the miracle of restoring the political life to the cold and decaying flesh of his career, it is all up and resignation is just a matter of time.

At which stage is the political career of David Cameron? We are well past the unspoken thought stage. And the whispering has given way to very loud and public conversations. The open resistance has hardened into an outright rebellion, and in several major policy areas David Cameron has had to rely on his political enemies to outvote his erstwhile political friends. Beyond the stage of rebellion we know that 25 of the 46 MPs necessary to initiate a leadership challenge have already submitted letters to Graham Brady, the chairman of the 1922 Committee.

Of these 25 names, two are known to us. Patrick Mercer, MP, has found himself eliminated from the process by his losing the Conservative Whip. And the other name is Andrew Bridgen, MP, who has raised his head above the parapet and has already begun to suffer the sniping from the Cameron trenches across No-man's land. It doesn't even matter who could be promoted as a possible successor to Cameron. The fact that it is now a matter of fact that life after Cameron is being considered is enough to fatally undermine him. MPs now have a clear choice. They can continue to try to persuade and convince a Prime Minister who is not listening to them and prefers the support of the Labour and Liberal Democrats. Or they can write a brief letter to Graham Brady and add their names to the growing list of those who recognise that Cameron is now less popular than his Party, and that Cameron's actions and obsessions are likely to lead to many Conservative MPs losing their seats.

The end is nigh. However long Cameron can hang on, he has the air of a dead man walking. When MPs are willing to discuss the succession out loud and in the press then it is already too late.