I feel sick to my stomach when I contemplate what a Donald Trump victory would tell us about Americans

In medical news far beyond any margin of error, the nausea is now permanent. Previously, it came in waves which ebbed and flowed with the latest polls. Two days before the US election, the queasiness is a constant.

Here – and this is unusual in a life blighted by psychosomatic ailments – I am not alone. Since the polls began to tighten early last week, a growing number of friends and relatives have been in daily touch for glib reassurance – “It is going to be all right, isn’t it? I mean, it can’t actually happen…” – just as I have turned to them for the same.

We are terrified to a degree and in a manner which, though it may ring a bell (or sound the four-minute siren) for those who lived through the Cuban missile crisis, is a novel experience for anyone under 65.

By no means is it due solely to his cavalier take on deploying nuclear missiles that the prospect of President Trump is uniquely terrifying. Any of multiple other concerns might necessitate a nip from the vodka bottle and/or the popping of an antiemetic each time a poll shows him edging ahead in Florida.

Some are specific fears about master plans he has either floated or pledged to see through (scrap Obamacare, start an invigorating trade war with China, default on the national debt, and so on). Others, such as his potential to inflame existing racial tensions into an inferno, are less precise.

Overarching all of them, other than the one shaped like a mushroom cloud, is that nebulous, brooding sense of an ending. A Trump victory would mean a change of direction from which America’s global pre-eminence might never find its way home. You can abhor the grotesque hypocrisies that have long infected US foreign policy and still regard it as the world’s best hope. That would die on Tuesday if Clinton goes down.

It needs no underlining that a Trump win would be an indelible, eternal stain on the US. How a pussy-grabbing, race-baiting, tangerine-hued pantomime ogre with the attention span of a labrador puppy, the moral sensibilities of a slum landlord, the verbal dexterity of a stroke victim, and the default vindictiveness of a mafia capo could be so close to such an office remains indescribably hard to believe.

And I use that adverb in its literal sense. Even with a language as rich as English, the thesaurus has nothing that adequately describes it. The closest I can come to conveying the horrified incredulity is to summon to mind the face in Munch’s “The Scream”.

Seventeen months after he declared his candidacy, the notion of Donald Trump as president feels as much like an acid-fuelled absurdist fantasy as it did the day he kick-started the freak show last June. Nothing begins to inure you.

Regardless of Tuesday’s result, it breaks the heart to smithereens that he is in contention. Last week, Jon Snow went to Washington DC and tracked down two young black women who had featured in Channel 4 News reports eight years ago. As teenagers in 2008, they were filmed whooping, shrieking and weeping when a network called the election for Barack Obama.

“If there is anyone out there who still doubts that America is a place where all things are possible,” Obama began his victory speech in a Chicago park, “Tonight is your answer.” It seems unlikely that his definition of “all things” stretched as far as being succeeded by Donald Trump.

“To all those watching tonight from beyond our shores, from parliaments and palaces, to those who are huddled around radios in the forgotten corners of the world,” he went on, expressing a thought that soared beyond repudiating the George W Bush past to offer the credible promise of a better future, “Our stories are singular, but our destiny is shared. A new dawn of American leadership is at hand.”

Eight years on, our destiny is still inextricably linked to the occupancy of the Oval Office, and once again a new dawn of American leadership is at hand. Has global democratic history ever thrown up as starkly Manichean a contrast as that between Obama and the creature who would replace him?

Ordinarily, if only to offer comfort, you’d caution against hysteria at a moment such as this. But there never has been a moment like this when a brittle, vengeful oaf who, during a national security briefing, asked thrice within an hour why a president cannot use nuclear warheads as first-strike weapons is within touching distance of acquiring the power to fire them.

The writer who ghost-wrote The Art of the Deal for Donald Trump, and knows his twisted psyche better than most, regards his election as a clear and present danger to the species.

The odds are very heavily against a Trumpocalypse. Barring systemic polling error, Hillary will probably eke out her win on Tuesday. Even if not, the eve of election petrifaction no doubt causes one to exaggerate the degree of nihilistic lunacy of which Trump is capable.

Yet even disregarding how nuclear trigger-happy he appears, there is a serious quantifiable chance – whether one quantifies it at 25 per cent like the bookies, or at 35 per cent like the polling guru Nate Silver – that America will take a giant step towards fascism on Tuesday. We are standing on the edge of a cliff, all of us, staring down into a very deep void. Small wonder that the vertigo is making us nauseous.