Watching Donald Trump’s final campaign ad “Argument For America” on my own in a glassy think-tank office in Washington DC, as two tired Latino cleaners began their after hour rounds, I began wondering – how will historians explain the rise of this American demagogue?

You can’t see it in Washington today, where almost every professional and office worker you meet is white, but I am sure these historians will certainly begin that America was in 2016 at an ethnic tipping point.

They will ignore the feel-good, ahistorical buzz of liberal America and place Trump squarely in the arc of a dark history. It will seem obvious to historians why the United States, a country founded as much on white supremacy in 1776 as on human rights, where the army remained segregated into black and white units until 1948, where 17 states practiced segregation until 1964, was one where in the 2010s millions of whites became anxious about their slide into minority status.

White Americans are projected to become a minority as early as 2042.

Donald Trump predicts US election win

In 1950, when Trump was four years old, the United States was 87.5 per cent non-Hispanic white, 10 per cent black and only 2.1 per cent Hispanic. By 2010, when Trump turned 64, the United States had become only 63.7 per cent non-Hispanic white, 16.3 per cent Hispanic and 12.3 per cent black.

No other developed state has ever undergone ethnic change so rapid.

All American figures mask much starker changes on ground. In 1980, nearly half of US counties were 98 per cent white. Today less than 5 per cent are.

Racial anxiety is deep in white American ethnicity.

Now Trump has weaponised it.

Historians will sum up the surging resentment, protest and truther movements that accompanied the election of the first black president as emanations of a deep-seated fear, even fury that power was shifting.

Historians will dwell on the still-obscure debates that exist today in the Republican Party between the reformers who favored a compromise on immigration – and thus ethnic change – to win over the Hispanic vote; and the reactionaries who wanted to suppress non-white turnout through blatant gerrymandering and drive up their white vote with brash identity politics.

Donald Trump abruptly answered this question for the Republican Party. This former registered Democrat began cynically promoting the idea that Barack Obama was “born in Africa” to underscore the fact he was black as ethnically the United States became a new country.

To understand what Makes America Great Again, you need to separate the signal from the noise – and his blathering noise is all over the place. The unspoken signal is crystal clear: keep America white.

Data confirms resentment of ethnic change fuelled Trump’s crushing victory in the primaries. Areas most unsettled by mass immigration were the ones most likely to back Trump. The last 15 years has seen America’s Hispanic geography grow dramatically. Midwestern towns, a decade ago nearly 100 per cent white, that have seen Hispanics sore into a sizeable minority in the last 15 years were the most likely to back Trump. These are precisely the states – Iowa, Michigan, Wisconsin, Ohio – he believes hold, for him, the keys to the White House.

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

Looking from the future at the trend lines, the collapse of Christian America is even starker than white America. Historians will argue this explains why evangelicals were willing to vote for a man who so clearly did not live by their values – as he promised to keep them dominant.

Hegemonic groups, as they shrink, are at their most vulnerable to populists. White American Christians are already a minority – the rise of American atheism means they are now only 47 per cent of the population and their share of the electorate is collapsing. In 1994, white Christians made up 74 per cent of American voters. By 2016, white Christians had shrunk to 58 per cent of the electorate. By 2024, they will be just another minority.

The fading of White Christian America made American conservatives extremely vulnerable to a demagogue. But historians will be definite his rise was not inevitable. Driving around three states and the District of Columbia, one hears over and over again that something else is at work.

Simply put, Trump’s propaganda is working.

Trump is now so inside American minds that wherever one turns, one endlessly hears his demand for a wall or to drain the swamp.

But why was it so easy for Trump to weaponise racial fear?

Historically, this is pretty easy to do.

Yet eating peanuts in front of my TV and checking Twitter for the news, I am now convinced historians will not be kind on American media for creating a landscape of propaganda. They will surely note that out of no plot, but wholly decentralised and in relentless pursuit of profit, celebrity hosts, websites, newspapers and talk radio stations and TV channels like Fox News began pumping out of a form of agitprop for profitable ratings.

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This delegitimised American institutions and radicalised their views.

Historians will be as castigating on Facebook and Twitter, which motivated solely by monetised traffic, created systems that promoted traffic – any traffic – which trapped readers in filter bubbles reflecting their own views, allowed fake news and hate speech to thrive, and refused to do anything about it lest it reduce the traffic they monetised.

This is how Fox News and Facebook were both unwittingly scorched the terrain for Donald Trump to vault over as a master of propaganda.