It will, we are confidently told, be the most important American election for generations. In the words last week of Dick Cheney, the voice of what passes for gravitas in the Bush Administration, Americans will have to make 'about as serious a decision as anybody is ever asked to make' when they go to the polls in 17 days' time.

The prophets of doom, whom Cheney exemplifies, are precisely right about the importance of this election. But the momentous decision awaiting Americans is not whether they return to power a President who is uniquely qualified to protect the US against terrorism, as Cheney et al would have us believe. It is whether they re-elect a man who, it is now clear, has become palpably unstable.

The evidence has been before our eyes for some time, but only during the course of this election campaign has it crystallised - just in time, possibly, for the 2 November election. The 43rd US President has always had a much-publicised knack for mangled syntax, but now George Bush often searches an agonisingly long time, sometimes in vain, for the right words. His mind simply blanks out at crucial times. He is prone, I am told, to foul-mouthed temper tantrums in the White House. His handlers now rarely allow him to speak an unscripted word in public.

Indeed, there are now several confusing faces to the US President, and we saw three of them in the live, televised Presidential debates with John Kerry that culminated last Wednesday night in Tempe, Arizona. In the first debate on 30 September, watched by more than 62 million viewers, we saw Bush at his most unattractive: slouching, peevish, pouting, pursing his lips with disdain at what his opponent was saying. But he was unable to marshal any coherent arguments against Kerry and merely spewed out prepared talking points - in what, even his ardent supporters concede, was Bush's worst-ever such performance.

In the second debate on 8 October in St Louis, Bush could not stay on his stool and leapt up to dispense what were - certainly in contrast to Kerry's cogent recital of statistics and arguments - frequently defensive, shouting rants. I assume that he was told by his handlers not to show displeasure at Kerry's words this time around, but, instead, he revealed his anger by blinking repeatedly.

The moderator tried to stop him talking at one point (both campaign organisations had agreed the order in which the candidates could speak, with time limits imposed on both), but Bush insisted on riding roughshod over the briefly protesting moderator, Charles Gibson. (What, I wonder, would have happened if Gibson had kept to the rules and insisted that Bush stop talking? We will never know.)

By the time of the third debate on 13 October, this one witnessed by more than 50 million people, Bush had adopted yet another baffling persona. This time, he was peculiarly flushed, leading a colleague to speculate whether he was on something. He had clearly been told to look positive - that was his main thrust of the evening, with frequent assertions that 'freedom is on the march' - and spent the evening with a creepy, inane grin on his face, as though he was red-faced after a festive Christmas dinner.

So what is up with the US President, and why is this election so crucial not only for America but for the world? I have been examining videos of his first 1994 debate with Ann Richards, the Governor of Texas, who he was about to supplant, and of his 2000 debates with Al Gore. In his one and only debate with Richards a decade ago, Bush was fluent and disciplined; with Gore, he had lost some of that polish but was still articulate, with frequent invocations of his supposed 'compassionate conservatism'.

It is thus hard to avoid the conclusion that Bush's cognitive functioning is not, for some reason, what it once was. I am not qualified to say why this is so. It would not be surprising if he was under enormous stress, particularly after the 9/11 atrocities in 2001, and I gather this could explain much, if not everything.

But I have heard wild speculation in Washington that he is suffering from a neurological disorder, or that the years of alcoholism might finally be taking their toll on his brain.

I think it unlikely that Bush was wearing a bug so that he could be fed lines in at least one of the debates, but it is indicative of how his capabilities are regarded these days that the suggestion that he needed advice is given credence, as well as passing mentions in the powerful Washington Post and New York Times .

It does not help that Bush now lives in a positively Nixonian cocoon. He does not read newspapers; he sees television only to watch football; he makes election speeches exclusively at ticket-only events, and his courtiers consciously avoid giving him bad news. When he met John Kerry for their first bout on the debating platform, it was almost a new experience for the President to hear the voice of dissent.

A senior Republican, experienced and wise in the ways of Washington, told me last Friday that he does not necessarily accept that Bush is unstable, but what is clear, he added, is that he is now manifestly unfit to be President.

This, too, is a view that is widely felt, but seldom articulated and then only in private, within the Republican as well as Democratic establishments in Washington. Either way, the choice voters make on Tuesday fortnight should be obvious: whether he is unstable or merely unfit to be President - and I would argue that they amount to much the same - he should speedily be turfed out of office.

But Bush and his handlers like Cheney are driven, if nothing else, by a primal and overriding need to win, to destroy enemies who are blocking their way (shades, again, of Nixon?). Thus the speeches Bush now reads to the Republican faithful at his campaign meetings reflect their intent to demonise and annihilate Kerry's character in the eyes of the electorate; policy statements made by Kerry are wilfully distorted and then endlessly repeated so that, in the end, the distortions gain a credence among the majority who do not follow such matters closely.

Whether the American electorate choose to see the mounting, disturbing evidence about their President or whether they rally to Cheney's obscenely manipulative appeals for their patriotic support is still up in the air.

Kerry is a poor candidate who has only recently woken to the need to fight. Bush manages to maintain a peculiarly American, ordinary bloke image - mystifyingly so, given that he is the privileged product of Andover, Yale and Harvard - that still contrasts well, in the eyes of many Americans, with Kerry's patrician manner.

The polls taken since Wednesday night's debate are infuriatingly contradictory, too. The only consoling thought is that soon we should know the result of that very serious decision the American people have to make on polling day. There are not many occasions when I agree with anything that Dick Cheney says, but this is one of the rare moments when I concur totally with those chilling words.

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