LONDON — If you believe the U.K. election polls, Theresa May is on course for a landslide victory — and she will lose seats, leading to a hung parliament and messy coalition talks.

Evidently both can't be true.

The divergence in the projections from pollsters, with some predicting a commanding 15 point lead while others put the margin as low as 3 points, matters not just because it is making the election result hard to call. The key factor at the heart of the variation is also key to the election result itself — namely, whether younger and less well-off voters will turn out, and in what numbers.

To achieve an upset, Labour are going to be hugely dependent on the June 8 election bucking a well-worn trend: the one that says younger and poorer voters — who appear to be rallying behind party leader Jeremy Corbyn in droves — don’t actually vote come polling day.

To achieve an upset, Labour are going to be hugely dependent on the trend that says younger and poorer voters don’t actually vote come polling day.

In the 2015 general election, for example, 18 to 24 year olds were almost half as likely to vote as those aged over 65 (43 percent vs 78 percent). In the Brexit referendum the disparity was less pronounced but still significant — 64 percent of 18-24s registered to vote, compared with 90 percent of over 65s.

“In many ways, it has become the defining feature all of a sudden in the difference in the polls," said Martin Boon, director of ICM Research, "with us basically disbelieving — I guess is the right word — the sudden surge in turnout likelihood among those who traditionally have not turned out in great numbers, and those polling companies who have taken it at face value.”

Younger voters

Since the debacle of 2015, when all the pollsters called the election wrong, overstating the Labour vote and underestimating Conservative support, polling companies have been working on ways to improve their findings.

That has led to a variety of responses from different companies. The key difference now emerging between companies’ methods is how they model turnout; in other words, how likely they think the people they survey are to actually go out and vote, and how heavily their voting intention should therefore be "weighted" in the final findings.

Those polling companies — such as YouGov and Survation — that are finding the biggest Labour vote shares and the narrowest Tory leads are also the ones that place a heavier emphasis on self-reported likelihood to vote, experts said.

All companies are also taking heed of how demographic status — youth versus age, rich versus poor — has affected likelihood to vote in previous elections. But some, such as ComRes and ICM Research, are weighting their findings more heavily on these factors. No surprise then that these are the companies giving the Tories’ bigger leads.

Other variations in the way different pollsters conduct their surveys and choose their samples are at play, but turnout modeling now appears to be the key point of difference leading to divergence in the Conservatives' lead in different polls.

“Some of [the divergence in the polls] is undoubtedly to do with presumptions about turnout among younger voters,” said John Curtice, professor of politics at Strathclyde University and one of the U.K.'s most respected polling experts.

“My reading is that ICM and ComRes are making pretty tough assumptions, YouGov and Survation certainly making weaker assumptions, ORB in-between,” he said.

Boon of ICM Research agrees. “The companies which have remained with the old way are showing lower Tory leads and higher Labour shares. The companies like us that have adopted an assumed probability model are showing higher Tory leads and lower Labour shares,” he said.

Milifandom and Cleggmania

In short, if the Tory lead is to be as narrow as polls by YouGov and Survation suggest, the youth turnout is going to have to be big. The Survation poll, which gives the Tories a slender six-point lead, is based on 82 percent of 18-24 year-olds voting or having already voted.

“Either this requires a full rewriting of the psephological textbook or needs to be viewed with extreme caution,” Boon wrote in a blog post Tuesday. Some in Labour are also skeptical.

“I am a bit worried that we are in for what we saw in previous elections," said candidate Wes Streeting, who is fighting to retain his marginal seat of Ilford North. "With the so-called Milifandom or Cleggmania in the 2010 general election, you got a bit of excitement amongst groups of voters which lead to a poll lift, but then that is not reflected in actual votes on the day.”

But those close to the leader are talking up the chances of the party’s mass membership — which has more than doubled since 2015 — getting through to young voters that have remained untouched by previous campaigns.

A senior Labour campaign official said: “Our campaign has inspired huge numbers of young people. It’s absolutely vital that we turn that enthusiasm and excitement into votes in the ballot box on June 8. Our huge on-the-ground campaign, building on the work of our over half a million members will mean our efforts for getting out the vote will be larger than they’ve ever been before, and that should assist particularly in getting harder-to-reach groups, such as young people, out to vote.”

Driving the news

After June 8, we will be able to see which of the pollsters has come up with the best combination of new techniques post-2015.

One entirely new approach has made headlines. YouGov’s seat projection, published in the Times, estimated that if the vote were held today, the Conservatives would lose 20 seats, leading to a hung parliament.

It was based on an emerging new method called the multi-level regression and post-stratification model, or MRP for short.

It takes polling data, in YouGov’s case, from 50,000 interviews with registered voters over the course of a week, as well as modeling of key demographics and voting behavior for every constituency in the 2015 general election and last year's EU referendum. It then combines the two and makes a projection of how people are likely to vote in each seat.

It’s shock finding led to comparisons with 2015 polls which overestimated Labour’s lead, and have been blamed by some in the party for sowing complacency among Labour voters while driving Tory supporters to the ballot box.

Former Shadow Chancellor Ed Balls — who lost his seat in that election — said on Twitter it was a “concerning trend that polls now often driving the news,” while former Shadow Foreign Secretary Douglas Alexander, also a victim of that election, tweeted that journalists should treat the polls with “extreme caution.”

(11/11) Polls don't just reflect but shape a campaign. I offer this advice as the guy the 2015 polls said would be the next Foreign Secy. — Douglas Alexander (@D_G_Alexander) May 31, 2017

Joe Twyman, head of political and social research at YouGov, countered that collecting and publishing data throughout a campaign “is a good thing for democracy.”

“If you don’t you could only rely on canvass returns, speculation and pundits,” he told POLITICO. “We do not seek to influence. What we do is inform. And what if we had collated the data, seen the result and then thought: ‘that’s interesting, we shouldn’t publish that.' Wouldn’t that be bias?

“We do not seek to influence. What we do is inform" — YouGov's Joe Twyman

“The reason the polls often become the story is because the campaigns themselves are lacking any other story,” Twyman added. “If you’re tightly controlling a campaign and not allowing any members of the public to interact with the leaders, what are people going to write about? How are they going to fill the 24-hour news cycle?"

Referring to an incident in the 2001 general election when the then Deputy Prime Minister John Prescott took a swing at a protester who had just thrown an egg at him at point blank range, Twyman said: “It’s no longer the case that on the first day of campaigning the deputy prime minister punches someone in the face.”

Tom McTague and Annabelle Dickson contributed reporting.