BILL CLINTON is not the kind of politician for whom self-deprecation trips lightly off the tongue. In eight years of covering American politics I have never heard him compare himself unfavourably with anyone.

So when, unasked, the former President drew attention to one of his most famous political weaknesses during a rousing speech on the opening night of the Democratic convention in Boston on Monday, you knew that something was up. It was like watching Narcissus proudly display a hairy mole.

“During the Vietnam war, many young men, including the current President, the Vice-President — and me — could have gone to Vietnam, but didn’t. John Kerry came from a privileged background and could have avoided it too. Instead he said: ‘Send me’.”

True, Mr Clinton’s moment of self-reproach was also, characteristically, an oblique tilt at the current occupants of the White House. But its real intent was much cannier than that. It was, in his own terms, the most direct way he could find to address the issue that will probably be the determining factor in this presidential election — whether John Kerry has the character to be president at a time of national peril.

That, in short has been the challenge of the Democrats this week in Boston and it will be Mr Kerry’s main objective when he steps up to the podium tonight.

It is beyond dispute now that George W. Bush is in mortal political danger. If the November election were a referendum on the incumbent, he would almost certainly lose it. Instability in Iraq, the failure to find weapons of mass destruction and an economy that steadfastly refuses to roar back to life represent too many strikes against him.

But when they vote in November, Americans will be asked not if they want to replace Mr Bush, but if they want to replace Mr Bush with Mr Kerry. It is when confronted with that choice that the worm of doubt twists in the voter’s mind and the hand that is poised to sign Mr Bush’s political death warrant is stayed.

Mr Kerry’s main weakness is not that he is a bit boring, though he is. His speeches have been described as eye-crossingly dull. But voters have elected dull men before. Nor is his principal failing his aloofness, though he is that too. It is not even that he looks like an old-fashioned Massachusetts liberal in the mould of Ted Kennedy or Michael Dukakis — his record is more complex than that.

The real problem is precisely this complexity, and what it says about the character of Mr Kerry. Over a long career, the Massachusetts senator has certainly registered a disquieting tendency to shift positions, to vacillate, to argue both sides of the case. He has given the impression that having made a decision on a subject, he likes to go back and make another one just to make sure all angles are covered. These may be sound qualities in a lawyer or a philosophy professor, but in a war president they look like terminal weakness.

He was a strong voice in favour of free trade for years, but now says he would reconsider open trade deals with other countries. He voted against war in Iraq in 1991, after Saddam Hussein’s tanks had stormed through an American ally’s territory. But he voted in favour of war in 2002, on the controversial ground of pre-emptive need.

Most damagingly of all (according even to some of his own advisers), having voted for the 2002 war resolution, less than a year later he voted against a measure to give the necessary $87billionn funding to finance continued military operations there. His explanation, like all fateful cover-ups through history, was worse than the original crime: “I actually did vote for the $87billion before I voted against it,” he said.

Some of Mr Kerry’s inconsistency can be attributed to cynical political calculation. His vote on the $87billion came at the height of Howard Dean’s surge in the Democratic primary. The judgment was that it would have been political suicide for him to vote for it.

Cynical political opportunism over the nation’s defence might be deemed troubling enough. But, in fairness to Mr Kerry, his bigger problem seems to be that there is something much more honest about his indecision. He genuinely seems conflicted on great matters of policy and this often stems from exquisite tussles with a complex conscience.

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