It seems like long ago now, but there once was a time when the Labour Party had every reason to be fearful of Theresa May.

The new Prime Minister had walked into Downing Street with a plan to transform politics by colonising vast tracts of Labour territory. Standing on the steps of No 10, May said she stood for social justice and building one nation. She even ended her speech with a very new Labour claim: “We will make Britain a country that works not for a privileged few, but for every one of us.”

Her plan failed because she became the hostage of those in the Conservative Party who want to close Britain off to the rest of the world. When May called the 2017 general election it was specifically to overcome the opponents of a hard Brexit – to crush the saboteurs – and achieve the narrow, extreme rupture with the European Union she had once campaigned against. And we all know what happened next.

Instead of achieving the overwhelming victory she sought, she lost both her majority and her political authority. Contrary to all expectations, including possibly his own, Jeremy Corbyn picked up 30 seats and took Labour back to where it was in 2010.

But some of the most fashionable theories for Labour’s improved performance do not survive detailed scrutiny. Figures released recently by the British Election Study show that the so-called “Youthquake” - a disproportionate increase in turnout amongst the young – never really happened.

The idea that “Jezmania” exists among the general public is undermined by evidence showing Labour’s leader continues to lag behind an ever-more unpopular Theresa May as Britain’s preferred Prime Minister.

And claims that the public was wildly enthused by the chance to vote for a radical left-wing manifesto are also unlikely when, give or take a dash of nationalisation, it was pretty much a cut-and-paste of the policy offer Ed Miliband had made with significantly less success two years earlier. Far from being a revolutionary upheaval of the commanding heights of the economy, Labour in 2017 was even pledging to retain most of the Tories’ benefit cuts.

Instead, the Global Future analysis published today by Andrew Cooper is by far the most compelling and rigorous explanation that I have seen. He shows where old alliances are breaking down and new electoral coalitions are beginning to be formed. He outlines how the real division in British politics now is not between Left and Right so much as being open to the world or closed. And he explains why there was such a significant swing to Labour, and away from the Conservatives, among voters under the age of 45.

In the age of identity, people were not motivated to vote simply by retail offers such as free tuition fees but by a much deeper recognition of shared values on issues ranging from multiculturalism, immigration and our country’s future place in the world.

For many people, May’s forced conversion to hard Brexit placed her firmly on the wrong side of this divide between Open and Closed. The Conservatives became the party of restricting immigration, building borders and killing foxes. “If you believe you’re a citizen of the world, you’re a citizen of nowhere,” said the Prime Minister. “If you believe you’re a citizen of the world, you’re not a Tory”, heard millions of voters.

While the Conservatives doubled-down on chasing voters on the Closed side of the divide, Labour made extraordinary gains in seats like Canterbury, Kensington and Battersea – where Open voters made the crucial difference.

Such voters chose to back Labour at the last election because they had little choice: in most seats, supporting another party such as the Liberal Democrats or the Greens would have most likely helped deliver a Conservative majority.

But a huge question mark hangs over whether these voters will make the same calculation again.

Although Labour MPs, Labour members and Labour voters are overwhelming on the Open side of the divide, the party’s official position still supports Brexit and appears to place limiting immigration ahead of Britain’s economic prosperity.

The time is fast approaching when Labour will have to stop facing both ways and pick a side between Open or Closed. Either choice will see votes lost as well as gained because there is always a trade-off.

But I believe choosing to place ourselves on the Open side of the divide is in accordance with our historic internationalist values. It is also in the interests of our voters, whose jobs and security rely on being part of an outward-looking country. And it is the right choice strategically.

This is not just a question for the next election: the divide between Open and Closed will define politics for decades to come. As Global Future’s analysis shows, the long term demographic trend of our country’s politics is moving steadily towards one where Closed voters will be far outnumbered by those with Open values.

For those in my party who fear we will lose seats in some traditional heartland areas where people have Closed values, I can only point out that in all but a handful of these constituencies, the Labour vote itself already strongly leans towards Open.

But most importantly of all, the Labour Party will not succeed – or, indeed, deserve to succeed – if it carries on pretending to be something it is not. The Conservative Party is heading towards becoming an authentically Closed party because that is where the values of their ageing voters and membership predominantly lie. But, even if we wanted to, there is no plausible path for Labour to shift against openness, diversity, and free movement, or in favour of a narrow, closed nationalism.

Nor do I believe that sitting on the fence between Open and Closed will for much longer be a viable strategy for our party. Despite this week’s welcome shift towards a Customs Union the party is still a long way behind it’s voters and members, and in this era of internal party democracy, the membership’s tolerance cannot be taken for granted by a leadership that is triangulating on an issue so fundamental to our country’s future and our party’s principles.

The axis of British politics is rotating and, as it does so, it presents Labour with a huge opportunity. The route back to government is through the progressive, inclusive politics of our members and voters.

The Open side of this great debate will own the future. And if Labour gets the strategy right, our party can too.