The Prime Minister still has more cards than his critics — just. It is very hard to oust prime ministers. Incumbency gives them the initiative and requires challengers to stake their careers on the chance that they will succeed. That is why so few PMs have been forced out between elections. Even the aged Winston Churchill carried on for much longer than most of his Cabinet wanted. Of the 13 who have resigned, rather than been voted out of office, almost half went because of ill health or age.

Balfour jumped as his party split before electoral defeat; Asquith and Chamberlain went during wartime crises; Lloyd George was rejected by most of his coalition allies; an already ill Eden quit after the Suez fiasco; and Margaret Thatcher lost the support of a large minority of her MPs, and then her Cabinet. Tony Blair went a year earlier than he had originally wished under pressure from his successor’s allies.

The key in each case was authority. Those ousted lost the confidence of close colleagues. They were seen as out of touch, and no longer having a grip or sense of direction. Crudely, they moved from being winners to being seen as losers.

Nonetheless, incumbents are still hard to shift. Wilson saw off several coup attempts and John Major survived five years of a bitterly divided party. It requires not just a sufficient mass of discontented MPs, but also a willingness by Cabinet colleagues to confront the PM. That is always the trickiest part. When Home was forming his government in October 1963, he called the bluff of supporters of R. A. Butler and the refusal to serve of Macleod and Powell. Mrs Thatcher was finally doomed in 1990 when several members of her Cabinet told her to stand down.

What a beleaguered PM needs is a wow factor — something that grabs the initiative, such as John Major’s decision in 1995 to seek re-election as Tory leader, which bought him time, if not harmony. Gordon Brown has already tried that once, last October, when the return of Lord Mandelson silenced the Blairite critics.

After a very damaging two days, does Mr Brown any longer have the authority to mount a credible reshuffle? Will people refuse the jobs offered? The key here is Alistair Darling, who has been a loyal, and remarkably patient, ally for so many years. If he is shifted from the Treasury, will he accept another post?

The risk for Mr Brown is if he is snubbed in his choices and can only construct a Cabinet dominated by cronies. Ed Balls understands the Treasury’s agenda, but he would be a big political gamble. Mr Brown wants to follow the reshuffle by a relaunch of plans for economic recovery, public service reform and political renewal. It may be too late for that. Mr Brown’s options are narrowing by the hour. But he still has a few left.

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