Rupert Murdoch brokered a "tentative truce" between Barack Obama, the presidential candidate who has won his enthusiastic backing, and the head of Fox News, the influential rightwing network he owns that has regularly pilloried the Democratic nominee, the media mogul's biographer has revealed.

Michael Wolff, a US media commentator who enjoyed unprecedented access to the 77-year-old News Corp chairman, describes in an article for Vanity Fair how Murdoch's meeting with Roger Ailes descended into a furious dispute before resulting in a "tentative truce".

He outlines how Murdoch has enthusiastically courted Obama even though the Democratic candidate repeatedly snubbed his advances, and this despite senior News Corp executives having recruited the Kennedys to act as go-betweens.

This marked a "leap" for the mogul - whose empire stretches from the Sun, the Times and BSkyB in the UK to 20th Century Fox, the New York Post and the Wall Street Journal in the US, and Star TV in Asia - because "Murdoch has traditionally liked politicians to come to him".

Wolff relates that Obama finally relented early in the summer and the pair met with Ailes at the Waldorf-Astoria hotel in New York.

According to Wolff's account, the meeting began cordially, with Obama asking Murdoch about his relationship with his father and his path from a single Adelaide newspaper to global media magnate. In turn, the mogul imparted his view that, having known possibly as many heads of state as anyone living today and met every US president since Harry Truman, nobody had much time to make an impression - "leadership was about what you did in the first six months".

But as Murdoch switched places with Ailes, Obama "lit into" the Fox News man. "He said that he didn't want to waste his time talking to Ailes if Fox was just going to continue to abuse him and his wife, that Fox had relentlessly portrayed him as suspicious, foreign, fearsome - just short of a terrorist," wrote Wolff, who was not present at the meeting but said yesterday his detailed account was correct "to the very best of my knowledge".

"Ailes, unruffled, said it might not have been this way if Obama had more willingly come on the air instead of so often giving Fox the back of his hand. A tentative truce, which may or may not have vast historical significance, was at that moment agreed upon."

"Murdoch is strongly, if not firmly, in the Obama camp. How that plays out on Fox is another question," Wolff said yesterday. "He actually has relatively little ability to affect what's on Fox, assuming he doesn't want to fire Roger Ailes, which I don't think he does. But I do think he would prefer him to be a little less unreconstructed."

The opinion-led US news network became intrinsically linked with the Bush administration and its key pundits have enthusiastically backed John McCain.

In the article, to be published on Friday, Wolff says Murdoch, partly due to the influence of his third wife Wendi, is "becoming a liberal - sort of".

"Fox has been his alter ego. For a long time he was in love with the Fox chief, Roger Ailes, because he was even more Murdoch than Murdoch. And yet now the embarrassment can't be missed - he mumbles even more than usual when called on to justify it; he barely pretends to hide the way he feels about [Fox pundit] Bill O'Reilly. And while it is not possible that he would give Fox up - because the money is the money; success trumps all - in the larger sense of who he is, he seems to want to hedge his bets."

The article, based on almost weekly interviews with Murdoch over a nine-month period, also paints a revealing portrait of Murdoch's love of mischievous gossip, his closeness to his children, his visceral passion for newspapers and his unsated ambition to add to his empire.

Wolff, a Vanity Fair contributing editor who wrote a book about his experience of the dotcom bust before reinventing himself as a media pundit, also believes Murdoch is serious in his ambition to buy the New York Times, the liberal totem that has cast him as "bogeyman and vulgarian".

But when Wolff asks Murdoch in their final interview what he can do to prevent what happened to the Bancrofts, the old Wall Street Journal dynasty, from happening to his family, he throws his hands up in the air and says: "Oh, simple, I can't. All I can do is delay it."