It is possible to sense relief, even guarded optimism, among US Democrats and right-thinking people everywhere. Donald Trump, the tycoon and political neophyte, who swept to the Republican nomination despite the hostility of party stalwarts, has had a bad couple of weeks, very bad, in fact.

Trump’s showing in the opinion polls has been inexorably downwards since Hillary Clinton’s triumph at the Democratic National Convention. He has reshuffled his top team for what seems the umpteenth time, and muttering can be heard to the effect that he never really wanted the job anyway. Wishful thinking even has sundry Republicans asking whether Trump might decide to make way for someone supposedly more electable.

Of course, it’s not over until it’s over. Trump’s whole campaign to date has defied the odds, and his camp insist – quite rightly – that it is very early days. The campaign proper never really begins until after Labour Day, the first Monday in September. Even then it will have two full months to run. What is more, there are legitimate questions about both the accuracy of polling and the volatility of US opinion. Americans vote on 8 November.

Donald Trump's most controversial quotes Show all 14 1 /14 Donald Trump's most controversial quotes Donald Trump's most controversial quotes On Isis: "Some of the candidates, they went in and didn’t know the air conditioner didn’t work and sweated like dogs, and they didn’t know the room was too big because they didn’t have anybody there. How are they going to beat ISIS?" Getty Donald Trump's most controversial quotes On immigration: "I will build a great wall — and nobody builds walls better than me, believe me —and I’ll build them very inexpensively. I will build a great, great wall on our southern border, and I will make Mexico pay for that wall. Mark my words." Reuters Donald Trump's most controversial quotes On Free Trade: "Free trade is terrible. Free trade can be wonderful if you have smart people. But we have stupid people." PAUL J. RICHARDS | AFP | Getty Images Donald Trump's most controversial quotes On Mexicans: "When Mexico sends its people, they’re not sending their best. They’re sending people that have lots of problems. They’re bringing drugs. They’re bringing crime. They’re rapists." Getty Donald Trump's most controversial quotes On China: "I just sold an apartment for $15 million to somebody from China. Am I supposed to dislike them?... I love China. The biggest bank in the world is from China. You know where their United States headquarters is located? In this building, in Trump Tower." Getty Images Donald Trump's most controversial quotes On work: "If you're interested in 'balancing' work and pleasure, stop trying to balance them. Instead make your work more pleasurable." AP Donald Trump's most controversial quotes On success: "What separates the winners from the losers is how a person reacts to each new twist of fate." Donald Trump's most controversial quotes On life: "Everything in life is luck." AFP Donald Trump's most controversial quotes On ambition: "You have to think anyway, so why not think big?" Getty Images Donald Trump's most controversial quotes On his opponents: "Bush is totally in favour of Common Core. I don't see how he can possibly get the nomination. He's weak on immigration. He's in favour of Common Core. How the hell can you vote for this guy? You just can't do it." Reuters Donald Trump's most controversial quotes On Obamacare: "You have to be hit by a tractor, literally, a tractor, to use it, because the deductibles are so high. It's virtually useless. And remember the $5 billion web site?... I have so many web sites, I have them all over the place. I hire people, they do a web site. It costs me $3." Getty Images Donald Trump's most controversial quotes On Barack Obama: "Obama is going to be out playing golf. He might be on one of my courses. I would invite him. I have the best courses in the world. I have one right next to the White House." PA Donald Trump's most controversial quotes On himself: "Love him or hate him, Trump is a man who is certain about what he wants and sets out to get it, no holds barred. Women find his power almost as much of a turn-on as his money." Getty Images Donald Trump's most controversial quotes On America: "The American Dream is dead. But if I get elected president I will bring it back bigger and better and stronger than ever before and we will make America great again." GETTY

Throughout this time, we Europeans will be commenting, wide-eyed, from the margins, as we always do, comparing our variegated systems with theirs, and almost entirely to our advantage. US presidential campaigns, we will say, are far too long, far too expensive and borderline corrupt. Even the voting system, with its differences between states, those ancient machines that frustrate recounts (remember the “hanging chads” of 2000), and the inequalities seemingly built into the registration system will be cited as proof of our superiority. Many Americans would agree; they are some of the least sparing critics of their own goings-on.

What Donald Trump’s unpredicted rise and – current – fall already show, however, is something we Europeans tend to neglect. What looks from our side of the Atlantic like a mad electoral marathon that serves no one well and costs extortionate amounts of cash is not a bad test of presidential potential. What is more or less a year-long campaign has a pace and a logic that ruthlessly exposes those who cannot make the grade.

A US election campaign has something of the medieval tournament about it. It offers a gruelling series of obstacles to be surmounted and trials to be endured. Whoever wins will have proved to be equal to the task of presiding over the most powerful country in the world. And yes, by the way, it still is.

The most basic requirement is stamina. Would-be presidents have to put in the time and the miles. Trump and Clinton have now been on the road more or less continuously since last autumn. They cannot be seen to flag. From September they will be crisscrossing the country, speaking at venues from vast sport stadiums to tiny parish halls, fitting in several meetings in one day. It is an intense replication of what it is like to be President of the United States. If you don’t like it, or can’t cope with it, the job is not for you. The last frantic dash, coast-to-coast, battle-ground to battle-ground is one of the ultimate in electoral challenges, and – in a close race – one of the most thrilling.

The second is the capacity to raise money; a lot of it. As Europeans we tend to find this distasteful – without acknowledging that party fund-raising has a tendency to be tainted, even in relatively “clean” countries. Like it or not, though, the role of corporate money in the United States makes the confidence of business a pre-requisite for any US president.

All Trump's lawyer can say in response to bad polling numbers is 'Says who?'

There are different ways to acquire it. Hillary Clinton has the benefit of her husband’s presidential network, and her own from her time as New York Senator and Secretary of State – corporate support which risks also being a liability this electoral year, because it associates her so closely with “the establishment”. George W Bush had the Bush networks and his own business ties. Donald Trump can boast – as could Michael Bloomberg – that his money is largely his own, and his business record stands for itself. Barack Obama’s biggest task eight years ago was arguably to convince corporate America that he was a winner.

As the personification of the state, a US President must also be at least, on the presentational front, competent. Bill Clinton was the master communicator, he grasps at once the disposition of his audience. He makes the connection, conveys the message, wins the argument – and all with an ease that his wife has had to learn. A future president must convince, both in person and, crucially, on television – even in this time of social media. If he or she isn’t born with the ability, they acquire it over the months of campaigning, with televised debates, even now, the duels that can make or break it all.

A US presidential campaign is the longest and most demanding political stress test conducted anywhere, and it's always in the public eye. Candidates must keep cool under pressure and cope with reversals. Personal flaws, past and present, can surface at any time. These need not be fatal, as Bill Clinton and Trump – so far – showed. It depends how the candidate handles them and how the public responds. John Edwards, the hapless presidential hopeful from North Carolina eight years ago will long stand as a model of how not to do it. Imagine if he had become President, and you understand the point of a US-style campaign.