Donald Rumsfeld's admission that there is no "hard evidence" that Saddam Hussein's regime was in league with al-Qa'ida, will have surprised no one who took the trouble to read the report produced by the US 9/11 commission in July. The commission concluded that there was no operational link between the Iraqi president and Osama bin Laden, despite the Bush administration's repeated insinuations to that effect. Mr Rumsfeld's hasty efforts to backtrack, by citing past CIA reports positing a Saddam-Bin Laden link, hardly convinced.

Donald Rumsfeld's admission that there is no "hard evidence" that Saddam Hussein's regime was in league with al-Qa'ida, will have surprised no one who took the trouble to read the report produced by the US 9/11 commission in July. The commission concluded that there was no operational link between the Iraqi president and Osama bin Laden, despite the Bush administration's repeated insinuations to that effect. Mr Rumsfeld's hasty efforts to backtrack, by citing past CIA reports positing a Saddam-Bin Laden link, hardly convinced.

In electoral terms, Mr Rumsfeld's was a gaffe of the highest order. But it was also the truth. And its significance is not just that the Secretary of Defence has become the highest-ranking figure in the Bush administration to admit it, but that US voters are almost for the first time taking serious notice. The war is now the central issue in the campaign and President Bush is, at last, being held to account for the weak - and highly ideological - case he made for deposing Saddam Hussein.

Mr Rumsfeld's embarrassing admission came as Dick Cheney and John Edwards made final preparations for their vice-presidential debate and it was bound to cast a shadow. It was, after all, in the first presidential debate last week that John Kerry scored his first real points against Mr Bush on the war. When Mr Bush denied that the war on Iraq was pre-emptive - because "they attacked us" - Mr Kerry pointed out that it was not Saddam Hussein who had attacked the US, but Bin Laden. Mr Bush could only yelp back: "I know that."

This was not Mr Bush's finest hour and, for once, America seemed to be paying heed. Mr Kerry clearly won the debate and, after lagging in the polls since his party convention, is now neck and neck with the President.

Mr Rumsfeld's frantic efforts to limit the damage yesterday suggest, too, that his mis-timed candour was more than a gaffe. It was the first real sign that Mr Bush's highly polished and well-oiled machine conceals a certain fragility. Under pressure, it could crack. Now that they have identified where Mr Bush is vulnerable, Mr Kerry and Mr Edwards must keep up the attack.