Five years ago, as he prepared to fight the general election of May 2010, the then prime minister, Gordon Brown, instructed officials to write him a particularly risky speech. It attacked the press. More than that, it attacked Rupert and James Murdoch by name, defending the BBC against their constant sniping, ridiculing the younger Murdoch’s obsession with profit, and calling for a new approach to the regulation of their papers. “It was a shot across the Murdoch bows,” according to one of the officials who worked on it. But Brown never delivered it.

Since April 1979, no British government has been elected without the support of Rupert Murdoch. That does not necessarily mean that it was Murdoch who won each battle. The facts and figures are too complex for such a simple conclusion. What it means is that politicians fear that his power may possibly be decisive, and so they play safe with him. At least, they persuade themselves that they are safe.

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Gordon Brown was rewarded for his silence with a bombardment of falsehood, distortion and spite from the Sun. There is nothing controversial about a newspaper using its leader columns to declare its position during an election campaign. This was different. This was a national newspaper that nine months before the election had embarked on a concerted effort to oust a sitting prime minister from office and which was not about to be deflected. Safety was simply not on offer.

What is less obvious is that life is not so much safer for those who enjoy Murdoch’s favour. For sure, they have the support of a news machine that is happy to treat its readers as ballot fodder. Murdoch’s UK chief executive, Rebekah Brooks, made her position plain in a text to David Cameron as he prepared to make his last party conference speech before the May 2010 election: “I am so rooting for you, not just as a personal friend but because professionally we are in this together. Speech of your life? Yes he Cam!” The Sun duly reflected her enthusiasm. For many months.

The core difficulty for any leader who engages in an electoral romance with the Murdochs is that the courtship will involve their adopting, or at least pretending to adopt, positions that may please the old mogul and his editors but which are going to choke the electorate as a whole. For instance, Cameron five years ago reacted to a loud chorus of complaint from the Murdoch camp about Ofcom by announcing that if he became prime minister, he would simply abolish the broadcast regulator. That thrilled James Murdoch, then the chief executive of BSkyB, who had been publicly humiliated twice by Ofcom obstructing his plans. But once Cameron was inside Downing Street, he found his promise was far too brazen a political gift to be delivered. The Murdochs were allowed to enjoy the sight of Ofcom losing 28% of its budget, nearly 20% of its staff and some of its most important legal powers, but they were denied a killing.

There were similar problems with some of Cameron’s wilder pro-Murdoch promises to cut or kill the BBC licence fee, which had to be toned down when it became clear that the director general, Mark Thompson, the chair of the BBC Trust, Sir Michael Lyons, and every other member of the trust were ready to resign together in protest if he persisted. And notoriously the Cameron government’s efforts to help Murdoch buy all of BSkyB very nearly cost his media secretary, Jeremy Hunt, his job when the Leveson inquiry disclosed that while Hunt was supposedly acting in an impartial “quasi-judicial role” he was allowing his special adviser to act as a back channel, supplying confidential information to the Murdoch camp.

And, of course, the phone-hacking scandal produced a special kind of jeopardy for a prime minister who had hired a right-hand man who delivered the benefit of being a member of Murdoch’s inner circle but who brought with him the problem of his past. That jeopardy was obvious when Cameron and George Osborne first approached Andy Coulson back in May 2007. They simply ignored the red warning lights: the potential benefit on offer was too tempting.

Politicians may as well protest against news companies that act as if it were their job to decide who runs the country

It took Cameron a long time to recognise that his wedding to Murdoch was dangerous. During his first 15 months in Downing Street, he held 26 meetings with Murdoch or his lieutenants. As the truth about the News of the World emerged, he steadfastly ignored it, preferring to stay close to Murdoch and to leave Coulson in charge of his government’s communications with the country. It was only when the scandal finally exploded with overwhelming political force, in July 2011, that he walked out on the relationship, calling on Rebekah Brooks to resign and setting up the Leveson inquiry.

A man who worked closely with Rupert Murdoch for years says: “Rupert is very loyal … until he isn’t any more.” There is no sign of his retaining any loyalty to his disobedient prime minister. Nor is there the faintest glimmer of affection for Ed Miliband, who had reached only the earliest stage of attempted hand-holding before speaking out against the hacking and – much more serious – organising the sabotage of the Murdoch bid for BSkyB. A senior Murdoch journalist texted advisers in the Labour leader’s office at the time: “Now it’s personal between us” and “I find it hard to see how you can do business with us after this”.

So, Cameron and Miliband and anybody else who fancies themselves as a political leader might as well speak out – to protest against news organisations that print propaganda and call it journalism, who are happy to smear and to expose the sex lives of those who dare oppose them, who behave as though it were their job to decide who runs the country, who after all the scandal and all the exposure of their crimes and abuse of power still enjoy the prerogative of harlots. What do those politicians have to lose? Nothing but the chains of fear.

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• Nick Davies’s book, Hack Attack, is now available in paperback