Oddly enough, Mr Gordon Brown has had quite a good week. He has held a Labour seat at a by-election, admittedly a safe one, but these days anything can happen. He has proposed a new tax on undeserved gains in the City to virtually universal applause. And he has been attacked in The Sun newspaper, so winning the sympathy of most of the audience. It will not make the slightest difference. Once trust goes, the Prime Minister – any prime minister – is finished. That is the accepted wisdom of the moment, and in Mr Brown's case it is probably right. Even so, the convention deserves closer examination.

Harold Wilson lost the trust of the electorate with the devaluation of 1967. He was described as a "twister" whose words could not be relied on. Wilson himself boasted that he had been more vilified than any prime minister since Lloyd George. And yet, two-and-a-half years afterwards, Wilson was the overwhelming favourite to win the election. It was a great surprise – at any rate, to the political classes – when Edward Heath entered No 10.

John Major had a whole five years, not of being distrusted exactly, but of being thought not up to the job. This was the consequence of our forced exit from the European Monetary System. As things turned out, the expulsion, far from being a national humiliation, proved beneficial to the Tory government and the new chancellor, Mr Kenneth Clarke. It made no difference, and the Tories were duly crushed.

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Tony Blair lost trust at some point in 2003 or 2004, chiefly on account of the Iraq war. But the voters concluded that the Conservatives were still a hopeless lot, with what was then their third leader since 1997 in place. Mr Blair came back with a reduced but perfectly healthy majority.

I do not think there will be much of a sympathy vote for Mr Brown. At the same time, there is a long history of voters who are reluctant to accept the instructions of newspapers. In the 1951 election the Daily Mirror asked the question "Whose Finger on the Trigger?" in relation to Winston Churchill. The Tories won.

More recently the same paper attacked "Tory toffs" on the basis that Mr David Cameron and his entourage were rich, had been to Eton or what-have-you. It made no difference. For a time the Labour Party's propaganda machine, such as it is – a far cry from Lord Mandelson's gleaming mechanism, which he supervised in his younger days – took up the slogan. But it seems to have been dropped.

Lord Mandelson has become increasingly tetchy at the withdrawal of the support of Mr Rupert Murdoch. Or, rather, it has been the support of The Sun, and not of his other newspapers, that has been withdrawn. Perhaps the others, or some of them, will follow. Who can tell? Who indeed! These are deep waters, my old friend. We are dealing with some desperate characters.

But it ill becomes Lord Mandelson to turn round and bite the several hands that nourished him – some of them, literally so – over a dozen or so years. The Lord giveth, and the Lord, or Mr Murdoch, has taken away.

Mr Brown, for his part, is not complaining so strenuously, or indeed at all. Mr Murdoch, as Mr Brown seems to be saying – it was a broadcast interview last Friday morning – has his job to do, while Mr Brown has his at No 10. If Mr Murdoch admires Mr Brown, Mr Brown has an even higher regard for Mr Murdoch.

The more worldly, even cynical,

comment has been that Mr Murdoch was seeking favours from the incoming Conservative government. The intermediary was Mr Cameron's press-and-media adviser Mr Andy Coulson, a former editor of one of Mr Murdoch's papers, the News of the World.

Some observers thought this a shrewd appointment by Mr Cameron; others considered it disreputable. What Mr Murdoch was seeking was the handicapping of the BBC and the continuing prosperity of Sky television and his other interests. So the story goes.

It is much simpler all round to conclude that Mr Murdoch believes in backing winners. The Sun arose from the ruins of the old Daily Herald, a paper jointly controlled by the Labour Party and the TUC. Mr Murdoch acquired the paper in 1969. It was, first, a Labour paper, but transferred its allegiance to Margaret Thatcher. It called her "Maggie" – she had hitherto been known as "Margaret" – and it made particular mock of Neil Kinnock. Lord Kinnock himself was later to attribute Labour's defeat in 1992 partly to the influence of The Sun.

The paper then turned on the new prime minister, John Major. The new hero was Mr Blair. This happy relationship continued until 2007, when Mr Blair gave way to Mr Brown. The paper (together with another of Mr Murdoch's productions, The Times) said that Mr Blair had been a great prime minister and that he was departing voluntarily, in his own time. This was only partially true, but that was what had been written at the time.

If Mr Blair had to deliver a speech to hard-pressed employees of Mr Murdoch's empire in some far-flung outpost of the globe, Mr Brown would not be far behind in getting on to the next convenient aeroplane, though he does not greatly like travelling by air. Moreover, Mr Murdoch admired Mr Brown, and still does, up to a point. He has a high regard for the virtues associated with Scotland, including industry and thrift, and also with his native Australia, such as an absence of affectation.

Why, then, did the paper attack Mr Brown with such ferocity when he had taken the trouble to write a personal letter to a bereaved mother? There were one or two misspellings, true. But even Evelyn Waugh was not strong at spelling, as he himself would confess.

In any organisation, there are those subordinates who are anxious to please the boss, often before they have been asked. Sometimes they guess wrong. They may even do the opposite of what the boss wanted them to do. This is particularly true of newspapers. "But this was what we thought you wanted." That is the plaintive cry.

It is one thing to withdraw support from Mr Brown; quite another to attack the Prime Minister so savagely and so needlessly; worst of all to lose the support of the public. The story goes (and I am in no position to confirm or deny it) that Mr Murdoch is displeased at the turn of last week's events. He would, it seems, have handled the story differently. His minions were guilty of an excess of zeal. My guess is that he will administer a rebuke at once earthy and lofty, if he has not already done so.

It must not be forgotten that persons of pomp and power tend to gang up together against the rest of the world. Thus the editor-in-chief of the Daily Mail, Mr Paul Dacre, professes the warmest regard for Mr Brown, while Mr Brown has the highest admiration for Mr Dacre. They have a chat together from time to time. The paper in question does not attack Mr Brown personally but reserves its intemperate indignation for the Labour Party and the Labour government.

There was a period – in the dying years of the last Tory government and then the new Labour administration – when Mr Murdoch's papers and even the Mail supported Mr Blair. Those days are long gone. And, some of us might say, not a moment too soon.