Winning the backing of 44 MPs sounds easy. It is for Mr Brown, and should be for John McDonnell in gaining support from the hard-left Campaign Group and other MPs who want a contest. But other contenders may find it harder to attract 44 MPs willing to declare publicly against Mr Brown. Some with ministerial ambitions will fear for their futures under Mr Brown.

A Brown succession still looks highly probable, but not yet certain. Doubts have been created by the failed coup by the Brownites three weeks ago, and by polls questioning his electability. That is why, behind the scenes, the talk has all been about the chances of alternatives, notably John Hutton, Alan Johnson and John Reid. Their allies have been testing the mood, while publicly being noncommittal.

At present, no one could beat Mr Brown. Timing is, however, the key. Rivals need longer to become better known, hence Mr Johnson’s pointed reference in his BBC Today interview to Mr Blair staying on longer than many expect. Mr Johnson has been the media favourite in recent weeks, but he and his views are largely unknown to many fellow ministers. The outcome mainly depends on attitudes to Mr Brown, and on whether he stumbles. He cannot afford any repeat of this month’s plotting, while he needs to show that he can reach out beyond his tight inner circle, be less tribal and more collegiate.

The other crucial test is electability. Even many who back Mr Brown are worried about how he will do against David Cameron. The same is also true of the largely unknown alternatives. There is likely to be a lot of attention on opinion polls and focus groups. These are likely be influential with MPs, especially in marginal seats. The crucial time will be when Mr Blair announces that he is stepping down, and the nomination process starts.

The deputy leadership contest is far more open. There is no front-runner. Indeed, there is uncertainty about the nature of the post. At present, the deputy Labour leader is also deputy prime minister. But there is no rule saying they should be the same, or that there has to be a Deputy PM. For long periods, there has been no one with that title. That is being considered in the Brown camp. One option is that the deputy Labour leader would also be party chairman, a post created in 2001, and so would have primarily a party role. Another possibility, raised by Jon Cruddas in launching his campaign yesterday, was that the deputy leader should not be a minister at all.

Defining the role is central to handicapping the race. Jack Straw, who is keeping his options open, is best qualified to be deputy prime minister, in the traditional sense of standing in for the prime minister, but not if it was just deputy party leader. Neither Peter Hain nor Harriet Harman would remotely qualify to be number two in the Cabinet, though both might attract some support for just a party role. Mr Cruddas is intriguingly placed to play a populist card, challenging current policies, which could attract union and party members. Mr Johnson could have broad appeal, as a senior minister but a relatively fresh face with a working-class union background.

But first, any candidate has to win over one in eight of Labour MPs — who will relish their nominating power.

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