The Sun has never handled politicians with kid gloves. During the exchange rate mechanism crisis in 1992, the then editor, Kelvin MacKenzie, famously told John Major: "I've got a large bucket of shit lying on my desk and tomorrow morning I'm going to pour it all over your head." But the paper's personal campaign against Gordon Brown marks a new level of vulgarity.

Its decision to attack Brown relentlessly over the spelling errors in a letter of condolence, with little or no reference to his damaged eyesight, was questionable enough, but it's the tabloid's persistent exploitation of a mother's grief for political purposes that is truly shameful.

There is no doubting the sincerity of Jacqui Janes, the mother of the dead soldier, but are we really to believe that she decided of her own volition to record her painfully awkward exchange with the Prime Minister?

The scribblers of Wapping would do well to listen to their former colleague George Pascoe-Watson, who has publicly expressed his concerns over the Sun's coverage. As John Rentoul reports, the red-top's former political editor said that it was "reasonable" to argue the paper was using Janes's grief to attack Brown and declared that there was no doubt Brown "cares passionately about the care of our troops".

I agree with those who say Brown's letter should have been given a quick once-over by a No 10 aide, but one could equally point to this as a refreshing departure from convention. A meticulously edited letter may have been more popular. It certainly would have been less personal.

It is something of an irony that this renewed assault on Brown should follow Rupert Murdoch's public expression of regret over the Sun's stance. In his most recent interview he said: "The editors in Britain, for instance, have turned very much against Gordon Brown, who is a friend of mine. I regret it."

Murdoch's words have been dismissed as a cynical front by Roy Greenslade and Michael White, but they reflect what Michael Wolff, author of the Murdoch biography The Man Who Owns the News, has long reported: that Murdoch has never been personally enthusiastic about the Sun's defection to the Tories and only nodded through the decision to keep his son and heir apparent, James Murdoch, onside.

As Wolff, who spent more than 50 hours interviewing Murdoch, wrote shortly after the Sun's announcement: "There may not be another politician in Rupert's nearly 60 years of helping to shoehorn the leaders of three countries into office who has personally appealed to him as much as Gordon. Rupert's voice changes when he talks about him. He gets ruminative (and Murdoch is not a ruminative man), and sentimental, and almost glassy-eyed."

It's partly a family thing: like Brown's father, the Reverend John Brown, Murdoch's paternal grandfather was a Scottish Presbyterian minister.

If he has any sense of dignity or loyalty, Murdoch should get on the phone to Wapping and call off this shameful, tawdry campaign.