The Democratic presidential primary contest has provided yet more proof, if any were needed, of the validity of Churchill's old quip that, while members of the other political party may be your opposition, your enemies are definitely on your own side.

In South Carolina this week, the two leading Democratic candidates squared off in what was by far the most testy and unpleasant series of exchanges of the whole presidential campaign so far. Barack Obama and Hillary Clinton spat accusations back and forth about the arcana of their various policy proposals and traded nastier gibes about who had been the bigger hypocrite in taking money from evil corporations while claiming to stand up for the little guy.

Then Mr Obama tried to shift the conversation to one of the many distortions by the Clinton team of things he had said on the campaign trail. Specifically he accused Mrs Clinton of misrepresenting some mildly positive remarks he made last week about Ronald Reagan.

“I did not mention (Reagan's) name,” Mrs Clinton said.

“Your husband did,” said Mr Obama

“Well, I'm here. He's not,” the former First Lady triumphantly shot back.

To which Mr Obama responded, almost ruefully: “OK. Well, I can't tell who I'm running against sometimes.”

It was a moment that captured not only the rapidly intensifying rancour of the Democratic race but the essence of what this presidential election is actually about.

If anyone ever doubted that the Hillary Clinton campaign was nothing less than a full-scale restoration of a political dynasty, the past two weeks will have surely disabused them.

Sixteen years ago, when her husband Bill first ran for president, he and his wife offered the US a famous bargain - “two for the price of one”. If they elected Bill, not only would they get the smart, roguish Rhodes scholar with the Southern drawl and the twinkling eyes. They would get, too, the cool, intellectually sharp, disciplined lawyer and policy wonk in her own right: Hillary - to many, the better half.

At the time the promise was regarded as a bit of a gaffe, a feminist leap too far for a country that liked its First Ladies to fuss about the gentler side of presidential labour - arranging seating plans for state dinners and espousing good, non-partisan causes such as tackling drug abuse and adult illiteracy. In their public rhetoric, at least, the Clintons dropped the idea.

But in office the plan went operational. Not only was Hillary a central figure in political decision-making, she was part of a symbiotic, co-dependent political relationship. Whenever Bill got into some scrape it was his wife's willingness to forgive - and go on the offensive against their political enemies - that saved his skin.

In her sharply insightful book, For Love of Politics, Sally Bedell Smith dissects the Clinton relationship, and says it is less like a traditional marriage and more like a vast and successful corporation that dominates the business of American politics. For eight years Bill was the President and Chief Executive Officer while Hillary was the top manager. When he left office, Hillary moved up to the CEO's suite and Bill took over as non-executive chairman. Now, the country is being invited to accept another takeover offer from Clinton Incorporated.

And so when the upstart young black senator from Illinois attempted to lead something of a shareholders' revolt against the proposal, he met the full force of the Clintons' wrath. Bill Clinton has been unleashed on the Obama candidacy like an ageing but still ferocious pitbull let loose on an elegant but slightly diffident Great Dane.

Starting when Mrs Clinton was at her low point just before the New Hampshire primary, the former President has sunk his teeth into the Obama hide and never let go. He dismissed Mr Obama's patently true claim that he had been the only candidate to firmly oppose the Iraq war from the start as “the biggest fairy tale I've ever seen”. In Nevada last week he virtually accused Mr Obama of rigging the vote in the Democratic caucuses there. And it was indeed, as Mrs Clinton gleefully pointed out in that South Carolina debate, her husband who has been accusing Mr Obama of the mortal Democratic sin of saying nice things about Ronald Reagan.

The spectacle has been unprecedented. When George W.Bush ran for president in 2000, his father, former President George H.W.Bush, who had himself been mauled by the Clintons in 1992, pointedly stayed out of the campaign. That might simply owe more to better breeding in the Brahmin Bushes from New England than you find in the dysfunctional family of street-brawling strivers from Arkansas. But it also true proof of how much is at stake for both Clintons this time.

Now you can think - as many Democrats do - that a return to the Clinton years is precisely what a battered and disillusioned America needs today. You can believe - as many independents do - that after seven years of a presidency that has done more to deplete America's strength and global standing than any in the past 30 years that it might not be so bad to venture back to those heady days when all anyone worried about was how to distribute the peace dividend and what to do with the budget surplus.

But Bill Clinton's behaviour these past couple of weeks ought by now to be flashing warning signs for American voters. It is not just that his egregious interventions in the campaign have revived the near certain prospect of a demoralising replay of the stomach-churning bitterness and vicious partisanship of the 1990s - not all of which was the Clintons' fault.

It is that, if nothing else, the history of America, from its very founding, has been a history of the struggle to constrain the appetites of powerful men - and occasionally women. It was this fear of the monarchical tendency that persuaded Americans more than 50 years ago to limit presidents to two terms in office.

By cleverly reincorporating themselves as a political institution in their own right, the Clintons are offering an extra-constitutional detour around this impediment. Her husband's evident role in his family's restoration should give any reasonable American pause when considering the virtues of Mrs Clinton's own claim to the presidency.

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