If Trump wins the Presidential debate the ensuing turmoil on the markets will make Brexit look like a sideshow

The Republican contender is a wildcard, and the markets are starting to feel fear as Clinton's poll lead narrows

If the man on the left wins the Presidential debate there could be a bloodbath on the markets (Nigel Parry for CNN)

He’s the ogre that stock market traders use to frighten their children into behaving. Be good or I’ll get my American friends to vote for Donald Trump, and there’ll be no place at a prestigious public school for you.

Markets across Europe took a bath in advance of the first Presidential debate. Some £22bn was wiped from the value of the FTSE 100 alone. There were corresponding falls across Europe.

The Donald isn’t the only thing giving traders sleepless nights right now. The continuing crisis at Deutsche Bank is almost as scary. Add them together and you’ve got the beginnings of a perfect storm.

Of course, the Donald will have to win for the fears to be realised. And that would appear to be quite the tall order given who he’s facing.

Hilary Clinton is a highly skilled debater and a master of policy who also happens to be as tough as nails. She’s very much a known quantity, beloved (and expansively funded) by Wall Street.

She’ll pay lip service to tightening regulation and instituting reform without changing anything much. She’ll be an effective administrator, who will try to help the USA’s struggling middle class, without doing anything to seriously scare the Street of Dreams or America’s government of money.

Then there’s Trump. Putin’s pal, who talks seriously about carpet bombing the Middle East, banning muslim immigration to the US and building a wall across the country’s border with Mexico. His proposed tax cuts might given even the billionaires they’ll most benefit pause for thought because they’d likely bankrupt Uncle Sam within a matter of months.

He’s a wildcard. He’s Brexit with bells on, with a finger on the nuclear button. The wonder is that only £22bn was knocked of the FTSE 100 in the run up to what could be a turning point in the campaign.

A turning point in what direction no one can tell. That’s the trouble with Trump. He’s defied every prediction up until now.

He’s managed to get away with things that would have destroyed any past Presidential bid.

Launching an attack on a Gold Star family whose son died in combat for the US, mocking a disabled reporter, suggesting that “there was blood coming out of her eyes, blood coming out of her wherever” when Fox News host Megyn Kelly refused to indulge in the sycophancy displayed towards the Republican by some of her colleagues. And there are so many more incidents like those. He should be dead in the water, but Trump is alive, and within sight of 1600 Pennsylvania Avenue.

American Presidential debates can, and have, had a major influence on Presidential elections. Everyone cites Kennedy vs Nixon, where the youthful Democrat provided such a stark contrast to the pasty faced and shifty looking Republican despite the fact that radio listeners, who weren’t able to see Tricky Dicky sweating, felt that he had won.

Then there was Ronald Reagan with his cheerful grin, his one liners and his “there you go agains”. He destroyed both Jimmy Carter and Walter Mondale.

There’s a lesson there for Hilary Clinton. Her mastery of detail, her meticulous preparation, and the fact that she will probably be able to slay Trump when matters of policy are raised as a result, might not matter.

It might not matter if Trump comes across as a brash and offensive bully, which has, after all, been his modus operandi up until now. He has managed to narrow Ms Clinton’s poll lead in spite of it all, in spite of the fact that even a lot of his supporters don’t like him very much.

Ms Clinton might just need to land a few low blows. If she does, call your spread betting firm, because the markets will shoot up. We’re saved! But watch out if she stumbles. If Trump starts to look like a winner - and I’ve written before that that is a very realistic possibility - then the turmoil on the stock exchange caused by Brexit will look like a sideshow.