We are standing on the brink of the abyss. And, like anyone who’s ever peered into a chasm, we are experiencing a queasy, sinking feeling. All round the world, not just in the United States, people are contemplating the prospect that on Wednesday morning we will wake to hear of victory for Donald J Trump.

The mere imagining of that outcome is inducing anxiety in those far away from the action. I don’t just mean obsessives such as me, who spend the midnight hour checking the early voting returns from Washoe County, Nevada. Otherwise normal people also confess to being reduced to nervous wrecks by the thought that Trump might actually win. They chart their mood swings on social media, delighting in hopeful news – Hillary Clinton up in ABC News tracking poll! – or panicking at any sign the snake-oil salesman might pull it off, such as today’s Washington Post headline: “Donald Trump has never been closer to the presidency than he is at this moment”, I could feel my palms turn clammy.

In Britain we feel especially vulnerable. If you voted remain, the memory of a ballot going the wrong way is fresh. And not just any ballot, but one you believe will cause lasting, epochal damage. The thought that Tuesday might bring the second such moment in a year is one to dread.

Thanks to Brexit, the usual reassurances – the expert endorsements, the polling data – have lost their calming properties. In June the smart money, including the betting markets, said remain had it in the bag. Burnt by that experience, nothing can soothe us now – except the right result. Until then, we have to chew our fingernails, hit refresh on the Real Clear Politics polling page and wait.

Many, especially in the US, will have a ready response: what’s it got to do with you? To which the answer is: plenty. The experience of the last photo-finish election – Bush v Gore in 2000 – taught many non-Americans a lesson we could not forget. Americans decide, but their decision affects the entire world. The supreme court’s installation of George W Bush as president had a profound and global impact. Just over a year later, Bush was agitating to invade Iraq, a choice whose consequences we live with still. (In Britain the focus is always on Tony Blair, as it was again this week when John Chilcot faced MPs. But that war would never have happened without Bush.)

It's the contempt Trump shows for democratic norms that has people fearing they are witness to something akin to fascism

So a President Trump will change lives far beyond the US. An American leader who believes climate change is a Chinese hoax, who believes terror suspects should be tortured and their family members killed, who believes that Saudi Arabia should have nuclear weapons, who is fascinated by nukes’ power of “devastation” and who has asked repeatedly why the US doesn’t use them; a man who says, “I love war”; a man who drools in admiration for Vladimir Putin and whose disregard for Nato, and refusal to promise to defend a member state if attacked, would all but invite Moscow to invade one of the Baltic states – such a man would plunge all of us into a dark future. That we are not living in the US will not protect us.

But his impact will be felt – is perhaps already being felt – in a more subtle way too. For like it or not, the US and Britain breathe some of the same air. Their politics rubs off on us. (This year, unusually, it’s worked the other way around too: Trump invokes Brexit daily.)

So a Trump presidency would exert a pull beyond America’s shores. Suspicion of migrants, loathing of Muslims, a desire to put up walls and roll back social progress – these currents exist everywhere. Were Trump to win, they would have the endorsement of the most powerful office in the world. For eight years, Barack Obama has been a cautionary voice, counselling against the global rush towards xenophobia and insularity. If Trump replaces him, white nationalism will have command of the world’s loudest megaphone. Racists and bigots everywhere will feel validated, vindicated – and mobilised.

The same is true of the contempt Trump shows for basic democratic norms. As much as the rank prejudice, it’s this that has serious people – including Republicans – fearing they are witness to something akin to fascism. His insistence that “I alone can fix” America’s problems; his threats to curb the free press, punishing news organisations that have criticised him; his hint that he would sack America’s generals and replace them with ones more compliant; his threats to jail his opponent and his winking hint that gun rights activists could find a way to deal with Clinton; his refusal to say he will accept the outcome of the election – with each of these steps, Trump has trampled on the foundation stones of liberal democracy.

Should he win, it would be a victory for a candidate who has lied more than any in history, who is spectacularly unqualified for the job and who stands contrary to the very idea of expertise. (Asked who he consulted on foreign policy, Trump answered, “I’m speaking with myself, number one, because I have a very good brain and I’ve said a lot of things.”) It would be a triumph over truth, facts and knowledge. It would be the start of a new age of endarkenment.

It sounds extreme, it sounds far-fetched. But that’s because we assume that stability, even the civilised order, are somehow the natural way of things, almost impossible to upend. But that’s not how it is. Civilisation is frail. The balances and restraints that hold us in check are delicate: they took many centuries to construct but would take only moments to smash. They rely on goodwill, trust and co-operation more than we realise. Take those things away, and darkness beckons.

In Britain, we are not smugly distant from this. Our own democratic system rests on respect for the rule of law and the judges who enforce it. Yet, even here a national newspaper can brand three senior judges “enemies of the people”, citing the fact that one of them is “openly gay” as evidence that their legal judgment was rigged and illegitimate.

This is Trumpism – he too attacks judges who won’t surrender to him – and it is a virus not confined to the US. Full of hatred towards anyone deemed an outsider, contemptuous of knowledge and red-faced with self-righteous fury at a nebulous, all-encompassing elite, it is becoming the movement of our times.

The first step towards its defeat is denying it a victory on Tuesday. Across the globe, we have to hope that happens – and that Americans end this nightmare before it can begin.