"The one to watch is Peter Mandelson. By bringing his former enemy back into the Cabinet last year Mr Brown locked in the modernisers and ensured his own survival. But if the Business Secretary were to turn on the Prime Minister (either in private or in public) no one could be more deadly. So far, there is no sign of that.
If, however, Mr Brown continues to ignore his advice, if he blocks his reforming plans for the Royal Mail, if he sidelines him in the run-up to the general election then the 'sinister minister' could be very dangerous indeed. 'Peter's fundamental view is that he's loyal to the Labour Party. The question is - is loyalty to the Labour Party the same as loyalty to the leader?' says an MP who has discussed it with him. 'That's not absolutely the case.'"
An intervention by Lord Mandelson would certainly carry some weight. On a symbolic level, he was returned to the Cabinet to reassure the Blairite faction, and to build bridges across the party. If he turns his back on Brown, then those bridges would be left smouldering. While, on a mythic level, Mandy is regarded as something of a kingmaker; someone who was instrumental in Blair's rise to power, and who could be instrumental in deciding the next Labour leader.
And what a grimly appropriate fullstop to Brown's career it would be. A man so often undone by his own actions, brought down by someone he pulled back into Government. Well, you can't say we didn't warn the Prime Minister.