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For millions of Britons their weekly shopping basket is revealing how they vote. For others, logging onto Facebook or Twitter means unknowingly disclosing their views to a new breed of number-crunchers taking over political campaigning. Welcome to the era of “big data” elections. And the biggest secret in politics right now is just how closely Britons are being monitored by those who want their vote in the London elections and in the summer in-out referendum on the European Union.

For the referendum you may already have been slotted into a category such as “head v heart”. If so, expect to be bombarded with bespoke letters, leaflets and emails in May and June. Or you may be deemed one of the “disinterested middle”, which means you will see a whole different set of tailored messages.

What makes it so different from traditional door-to-door canvassing is that the marketing experts behind the revolution no longer feel the need to actually ask you your opinion. Instead, armed with dozens of pieces of data such as social media habits, spending and postcode, they read your mind with startling accuracy.

The implications are huge. Not least because although there has been a quantum leap in political campaigning, the latest science of data analytics is not equally shared.

David Cameron has “the Knowledge”, which is partly why the Prime Minister managed to transform a somewhat mediocre 37 per cent vote share last May into an overall Commons majority and a place in the history books. No wonder a grateful Mr Cameron knighted the man who masterminded his electoral water-into-wine miracle, Lynton Crosby.

The In campaign for the European Union referendum also has it, having recruited many of the brains behind the Tory election campaign. But the Knowledge is jealously guarded by a few high priests.

And Zac Goldsmith has it, which is why polls showing his Labour rival Sadiq Khan seven points ahead may not reflect the Tory candidate’s real chances of moving into City Hall on May 5.

Jeremy Corbyn, however, does not have it. Neither does Mr Khan, whose energetic cross-London campaign could end up like Ed Miliband’s famous “five million conversations” that were supposed to deliver him victory last year but, to his surprise, lamentably failed. “We know that we are way, way behind the Conservatives on this,” says a senior member of Corbyn’s shadow cabinet.

Exactly how far behind is only just beginning to emerge, mainly because Mr Cameron’s referendum has brought together the three biggest political parties under one roof in the Stay campaign.

At first glance the open-plan offices of the Britain Stronger in Europe headquarters near Bank are a model of cross-party camaraderie, with Conservatives, Labour supporters and Liberal Democrats working closely together.

But all are not equal. The Evening Standard understands that even Will Straw, the campaign’s chief executive, does not have access to the secret methods by which big data is used to identify target voters.

“Will Straw just found out why he is not an MP,” boasts a Tory observer, wryly pointing out that Straw, son of Jack and regarded as a future Labour star, was one of the many Labour candidates who fought a key Labour target seat (Rossendale and Darwen, Tory majority of 5,654) only to find the Conservatives had somehow grabbed all the floating voters with a below-the-radar raid.

The chief priest of the Stronger In campaign is Andrew Cooper, the former No 10 strategist who founded polling firm Populus and was made a peer by Mr Cameron. Cooper is an expert on the mysterious art of segmentation: that is, dividing voters into small groups based on analysing fragments of publicly available data.

Another is Jim Messina, the Barack Obama 2012 campaign boss, who was paid £369,000 by the Conservatives at the 2015 election for advanced market research. Craig Elder and Tom Edmonds, leading experts in using Twitter and Facebook for political campaigning, worked closely with Messina on the Tory general election campaign. Elder and Messina are also advising Back Zac.

How data is obtained and processed is never divulged to outsiders. All Straw and the rest of his team are allowed to see are lists of names with numbers next to them. The numbers represent the segments that potential voters are deemed to belong to. No clues are given as to how each name came to be allocated with that particular number.

Much of the data can be bought off the shelf and was originally compiled for commercial marketing, including store loyalty cards and credit companies. Campaign experts hold close to their chest the wizardry by which they use it to split the millions of voters in micro-segments, tiny groups who share the same profile and views. Thus campaigns can be far more personalised than the traditional broad-brush appeals to age, social class and income, creating what US guru Bill Schmarzo calls “The Internet of One”.

An expert in the field explains: “Each person is assessed by around a dozen characteristics which include their social circumstances. From these you can extrapolate with considerable accuracy how they are likely to vote.

“First you can dispense with about 30 per cent who you know you can never reach. In the general election of 2015 the Conservatives identified 47 per cent that they would never reach and concentrated on the rest.”

The high priests will be targeting Head v Hearts most avidly. This is a segment who like the idea of British independence but have conerns over the economy and jobs. Ninety per cent are female; they do not use the BBC website and get most of their news from Facebook. The Disinterested Middles are a segment that do not bother with the news at all. But they could be targeted by social media.

Just how precise is the new science? An insight is given by a veteran Conservative who worked in a key south-west London marginal seat last May. “In the old days we were given a clipboard and entire streets to canvass door to door,” he said. “But this was completely different. I got into the passenger seat of a car and was driven to a single address. I had a script of what to say to a particular named voter who lived there. Then I got back in the car and was driven to a different address, where I had a different script for somebody else.”

The difference is between posters aimed at couples with young kids, and personal approaches to Mr and Mrs Smith who live in a key marginal, using doorstep persuasion, leaflets and letters. The most wooed voters in 2015 were known by the Tories as the “Yes, Yes, No” group — those who agreed that Cameron made a better PM than Miliband, agreed that the economy was improving but said “no” when asked if they would definitely vote Conservative.

The power of micro-electioneering is clear from the number of key seats that turned Tory on May 7 to the astonishment of Labour and Liberal Democrat candidates who truly believed they were safe.

The next test will come on May 5. Some Labour figures see parallels between the Khan campaign and Labour’s failed general election strategy. “Goldsmith is being criticised for looking complacent but so was Cameron last year,” said one. “Sadiq is praised for looking more energetic but so was Ed. I do wonder if it could all end the same way.”

Ironically it was a Labour leader, Tony Blair, who first embraced segmenting techniques to identity “Mondeo Man” and “Worcester Woman” in 1997 and 2001.

But the once-feared machine created by Peter Mandelson is rusty. “The Tories nowadays are something else,” said a source. “They know so much more about how people use social media. Labour’s guys look the part in their square glasses but while they are choosing between avatars the Tories can devise a completely new one.”

Thanks to the referendum, a few key people in Labour are waking up to the crisis. “The experts and the data are all available for a price,” said the shadow cabinet minister. “But it will be expensive.”

Whether Jeremy Corbyn agrees that he needs capitalist marketing rather than just Momentum may decide how many marginal seats he can recover out of those that slipped so unexpectedly from Ed Miliband’s grasp.