No matter how rock-solid your marketing, no matter how big your brand, the über-salesperson wastes no time in capitalising on the limitations of the opposition. Oprah said as much nine years ago when she was talking about Jerry Springer and what she called his vulgarity circus. “There comes a point when you have over-saturated yourself,” she noted, meaning that she did not see Springer as a threat to her own TV dominance. Might Oprah ever reach saturation point? Now it is being said that she has.

This conclusion has been mooted in the face of evidence from Oprah's empire that she is losing her appeal. Average audiences for The Oprah Winfrey Show have fallen by 7 per cent this year and, since 2004, viewing figures have dropped from 9 million to not much above 7 million. A second series of her prime-time philanthropy show has been abandoned and the circulation of O, the Oprah magazine, has fallen by 10 per cent in three years. Can this really relate to the woman who has mesmerised America for two decades, who has made her sisters comfortable with public confession and even persuaded them to read books? America can't quite believe it.

Other theories suggest that now she's 54, her audience is ageing with her, and that her endorsement of Barack Obama last October offended the white middle-class women who watch her show but also support Hillary Clinton. Was Oprah, long an effective social reformer over such issues as race and Aids, wise to reveal a political affiliation that could alienate a chunk of her audience?

Naturally, her office responds that business is thriving - and her show's status as the No1 talk show for 471 consecutive weeks is indisputable. Certainly, Oprah is not in the habit of revealing weakness. Her career has been predicated on rising above misfortune and this constant reference to the American Dream remains the basis of her appeal. Think of a victim group and Oprah has been there: female, black, fat, born out of wedlock in Mississippi, abandoned by her father, raped by a cousin at 9 and sexually abused by other men over the next five years until, at 14, she had a baby who died.

Winning a beauty contest got her TV attention and in Nashville, she became the first black woman to anchor the news. She was on her way, and once her emotional and apparently unscripted delivery got her to a chat show sofa in 1984, she was free to roll out the confession-laden style that would affect mainstream culture globally. No wonder CNN has called her the most powerful woman in the world.

She is America's first black billionairess, too, and has signed up to do the show until 2011. We know little of her personal life but what's clear is that, while her work relies on displaying intimate emotions, it is her professional life that validates her.

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