Carla Bruni Sarkozy may be many things - First Lady of France, former model, sometime folk chanteuse; but there is one thing she most definitely is not - a sister.

When it comes to her fellow woman, Bruni makes Monteverdi’s power-hungry antiheroine Poppea look like Germaine Greer. According to a new book, Carla and Nicolas: The True Story (on which Bruni, we are told, has collaborated extensively), she taunted Rachida Dati, the French Justice Minister and, as it happens, a friend of Sarkozy’s former wife, Cecilia, on a visit to the Elysée Palace. Strolling through the private apartments, she is said to have gestured towards Mr Sarkozy’s bed and remarked: “You’d have loved to occupy it, wouldn’t you?”

That she is happy to have such a vitriolic and unnecessary remark attributed to her tells us several things about Ms Bruni. First, she is territorial. This is her way of warning off any would-be rivals. Secondly, she fights dirty; and thirdly, like Poppea, she finds the power of her husband’s position as erotic as the man himself.

There are two types of politician’s wife. The first type marries her man before he becomes famous or powerful. She marries him before the grace-and-favour apartments in historical settings, before the banqueting dinners and state visits, before the helicopter on the lawn and the silent, waiting chauffeur. Michelle Obama is one such political wife, a woman who mentored her husband when he joined her law firm and watched, with mild amusement, as he began his career in politics. Sure, his advancement was a source of pride; but it was also exhausting and, at times, inconvenient.

Now that he is heading more urgently in the general direction of the White House, Mrs Obama will have a number of things on her mind. Not so much in what style to redecorate the kitchen, or whether to hire Wolfgang Puck or Daniel Boulud to cook breakfast in it. No; she will be contemplating the fact that she is going to have to make an appointment with her husband’s diary secretary just to discuss their holiday arrangements; and that if she wants to hook up with some old friends for supper, they’re going to need security clearance. It will change their lives, and not necessarily for the better. Only one thing is certain: their genuine allegiance to each other. After all, she loved him when he was just a big-eared geek lawyer, right?

Carla Bruni, I fear, is the other type of political wife. She hitched a ride on the bandwagon once it was well and truly rolling along. Or, to paraphrase Caroline Aherne’s brilliant comic interviewer, Mrs Merton: “So tell me, Madame Sarkozy, what was it that first attracted you to the enormously powerful (but not actually enormous) President of France?”

Bruni is the kind of female that Dolly Parton, bless her country-and-western heart, talks about in her timeless hit, Jolene: “Jolene, Jolene, Jolene, Jolene, I’m begging of you please don’t take my man,” she implores. “Please don’t take him just because you can . . . your beauty is beyond compare,” she adds, “with ivory skin and eyes of emerald green” (now isn’t that a coincidence?), before concluding, somewhat sadly: “And I cannot compete with you, Jolene.”

Every successful male politician has a Jolene waiting for him in the wings: a woman whose beauty flatters his ego and confirms his power; a woman who is flirtatious, fun, happy to fit around his hectic, tiring schedule. Easy, available, photogenic. Not someone who nags him because it’s his turn to put the kids to bed and he’s addressing a roomful of donors; or who’s in a bait because he had to miss his son’s sports day to host a delegation of unsmiling foreign businessmen. A woman who (regrettably, sir, whispers his press officer and his top aide) simply does not have the wherewithal (or the raw material) to toss her hair alluringly and flash a megawatt smile the second a photographer hoves into shot.

It is this kind of relationship, rooted in less heady times, that shields senior politicians from the madness of public life. For when you’re psychotic with lack of sleep, when your allies have deserted you and your enemies are massing, the one thing that’s going to stop you from pressing the big red button on the mad motherboard that will destroy all human life as we know it is the person who was there before any of this began; the person who knows that your favourite food is lemon meringue pie and that you really like polishing your shoes by way of relaxation - and who thinks neither is a story worth briefing out.

Crucially, someone who does not have a vested interest in keeping you in power come hell, full-scale inner-city riot or high water. Someone who doesn’t care about the limousine or the clothing allowance. Finally, someone who doesn’t define herself, proudly, by her ability to sleep with the person at the top. There are many ways to judge a politician; and one is by the quality of person they climb into bed with.

Sarah Vine is married to a Tory MP

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