DONALD TRUMP was facing an easy ride to the White House when he was matched against Joe Biden. Now he has to face off against Kamala Harris things look very different. Trump has good reason to be scared.

He might even lose, again, although it is quite likely he will claim he won, against all the evidence. But he knows now he is in a race.

US presidential elections are usually consequential, even if those consequences are often surprising. After all, it was not the non-WASP US golden boy JFK who ended the Vietnam war. That was forced on Tricky Dickie Nixon, who was later forced to resign from office in disgrace.

But it is quite possible that this could be one of the most consequential US presidential elections of modern history, which could affect every country on the planet. To understand the historical dynamics, we need to look into the background to the current contest.

When Trump beat Hillary Clinton to the presidency in 2016 he lost the popular vote by almost three million. This is because, as jurists from the US right now like to remind us, the US is not a democracy but a “constitutional republic.”

In the electoral process, this means each state casts votes for the presidential rivals in a 538-strong electoral college that takes place after the popular vote has taken place. Ever since its founding as the modern United States after the civil war, smaller states have had a disproportionate weight in the delegates to the electoral college.

Currently, these are skewed to the smaller, more rural or suburban states. But they are historically the smaller slave-owning states that were defeated in the civil war.

The constitutional republic is built on a compromise with slavery. This largely explains the anti-democratic nature of the system which means that the last Republican president to win the popular vote was George W Bush in 2004, having lost the popular vote but won the presidency in 2000 to Al Gore.

It was a massively increased turnout in 2020 that did for Trump, the first incumbent president to lose an election in almost 30 years. Joe Biden’s vote surged by almost 16 million votes compared to Hillary Clinton in 2016, an increase of almost a quarter.

It is one of the great ironies of modern US politics that Biden’s presidency was secured by the Black Lives Matter movement, despite the fact that his whole career had been one of complete opposition to the most modest attempts to combat racism in the US. Yet it was this factor and this movement which defeated Trump.

Black Lives Matter took off in 2013 after a series of cop killings of unarmed black people. Typically, no officers were ever charged, let alone convicted. These killings have become endemic in US society. Just this week Sonya Massey, an unarmed black woman, was shot dead by police officers in Chicago after she had called 911 to report a possible intruder. She was shot in her home while talking to the police officers.

Years of painstaking work by Black Lives Matter activists erupted into an enormous national movement after the killing of George Floyd, a movement with international resonance as we saw nightly, on our TV screens. It is estimated that up to 16 million people took part in one or more Black Lives Matter protests.

Floyd was another unarmed black man murdered in broad daylight by a cop. But, because of all that activism, and because it took place in an election year, it lit a touch paper of protest.

It is no exaggeration to say that 2020 was the Black Lives Matter election. The New York Times produces an excellent breakdown of the exit poll demographics for each national election. They are far superior to the various attempts here to shoe-horn data into a preconceived narrative. Think Ashcroft or various distortions of National Readership Surveys provide.

What the Times exit poll shows is that this was a “stop Trump’s racism” election: 90 per cent of black women voted for Biden and he easily led in every ethnic category except white (where he lost 58:41).

Racial inequality topped the list of voter concerns, identified by 92 per cent of Biden supporters. To underline why people came out in such great numbers, 53 per cent of Republicans said they voted mainly for Trump, while 68 per cent of Democrats said they voted against Trump.

By 2020, voters in general, but all ethnic minorities in particular, had seen enough of Trump to repel them. They either did not know or put to one side Biden’s history of allying with the Senate far right to oppose school desegregation or to incarcerate vastly more black and Latino men on minor charges.

In a similar fashion, the 2022 midterm elections were widely expected to be a rout of the Democrats in both houses of Congress. But “Stop Trump” again became the battle cry after the Supreme Court overturned Roe v Wade protections for a woman’s right to choose.

In these elections, strangely, Democrats lost the popular vote by a wide margin but won every single battleground seat. This was based on the differential in turnout of women. Essentially, where a Republican could be defeated, women turned out in their droves to oppose further curbs on the right to choose.

Which brings us up to date in 2024. Biden was always set to lose to Trump even before the shooting. He presided over the biggest decline in US living standards since the Great Depression.

He was politically incapable of galvanising the coalitions that had previously opposed Trump precisely because he agreed too much with him. On the economy, women’s rights, racism, trade union rights and war, Biden offered nothing different to Trump.

The question now is whether Harris can mobilise the same forces, or at least enough of them to successfully stop Trump. The short answer is yes, she can.

But it will not be easy. Political movements cannot just be conjured out of the air. And Harris may suffer by association with the deeply unpopular Biden. The more she can reasonably distance herself from her boss the better.

Opinion polls offer some encouragement, with Harris narrowly ahead since Biden announced he was stepping aside. But it is very tight, and certainly not enough to overcome the bias of the electoral college.

The stakes are enormously high and the consequences could be profound.

Diane Abbott is Labour MP for Hackney North and Stoke Newington.