For only now are pollsters starting to understand just how Trump has done it. And their findings will make tough reading for the business-as-usual politicians. Matthew MacWilliams, founder of a political communications firm, has been testing the factors that turn voters into Trump supporters. His surveys found the key wasn't race, age, income, church attendance, ideology or education: it was attitudes to authoritarianism. Donald Trump: "when he talks of his admiration for Vladimir Putin, plenty of Americans see not a gaffe but a reminder that a strong leader restored pride to a broken country". Credit:AP Donald Trump's supporters are looking for a strongman. Using that criterion, MacWilliams polled Republican voters in South Carolina last month and predicted that Trump would win with 33 per cent of the vote. Two weeks ago Trump actually won with 32.5 per cent. Not a bad indicator, then. Understand this, and the Trump discourse – strong versus weak, winners and losers, nativism, fear of the other – makes sense. It is an attractive message in 2016 America. Nothing characterises this election better than a creeping sense of insecurity: from jihadists launching lone wolf attacks in San Bernardino and Chattanooga to China's expansionist position on the other side of the globe. America is no longer the world's lone economic and military superpower. Many blame Barack Obama for diminishing, not burnishing, US global reach.

Beyond the liberal salons of New York and Washington lies a country wondering what happened to the American dream. Globalisation is taking jobs. The old deal – work hard and you will succeed – has been shattered by global economic collapse. Hillary Clinton: "she will have an almighty battle in the general election if she faces Donald Trump". Credit:AP So when Trump talks of his admiration for Vladimir Putin, plenty of American voters see not a gaffe but a reminder that a strong leader restored pride to a broken country. And when he describes that wall with Mexico, they see a man who will actually get things done – unlike Republicans who just talk of a secure border without saying quite how they would deliver it. So far, Trump has read the mood of the US far better than Republican suits and political journalists. But he is no ordinary demagogue. He has ripped up his party's orthodoxy, eschewing traditional appeals for small government and tinier taxes. He has publicly lauded Planned Parenthood – the abortion service hated by conservatives – and praised Britain's National Health Service. All of which makes him well-placed to pick off some of Clinton's supporters in a general election. The key lesson he has learnt is that America has polarised not between Left and Right but between those resistant or sympathetic to authority. Clinton's problem is that, being hawkish on foreign policy and pushing for a more thorough overhaul of healthcare, she is herself an authoritarian figure in an increasingly non-authoritarian party.

All of which leaves her facing an almighty battle in the general election if she faces Trump. Unlike her other possible opponents, he won't allow her the centre ground all to herself. And she risks losing some of her blue-collar backers to the other authoritarian in the race. Beyond the liberal salons of New York and Washington lies a country wondering what happened to the American dream. Internal Democrat polling obtained by the New York Post suggests the Clinton campaign is feeling the heat. She is even vulnerable against Trump in New York, her own backyard and one of the most liberal of electorates. Of course, it is not too late for her. Already she has begun positioning herself against her presumed opponent, repeating the phrase: "Instead of building walls we need to be tearing down barriers." And the Clinton-supporting fundraising organisations are starting to test their Trump attack advertisements. There is a long way to go in this election. But if Clinton wants to win, she has to understand what she is up against.

Donald Trump is no mere populist. Rather, he has spotted a defining shift in American politics. This campaign is already littered with the corpses of political heavyweights who arrogantly dismissed him as a reality TV star with funny hair and a quick line in insults. To their cost, they learnt that Trump is not a clown. He simply understands his country better that they do. Rob Crilly is a columnist with The Telegraph, London.