Prime Minister Theresa May's gamble on an early election no longer looks so clear cut. Credit:Getty Images As soon as the election was called on April 18, the Conservative numbers shot up. Pollsters quibbled over the size of the Tory landslide. While May tried to pretend she'd called the poll because of Brexit, the political calculation was clear. But in the last week, pollster YouGov has hit May with a double blow. First it did some seat-by-seat calculations and predicted the Conservatives would lose seats, Labour would gain, with a hung parliament the most likely outcome. Then on Thursday YouGov reported its latest polling figures: the gap between the parties was now three points: within the margin of error for the first time. Labour's rise in every poll – not just YouGov's – has been inexorable.

Labour Leader Jeremy Corbyn. Credit:Getty Images If YouGov's numbers are right (and they are disputed by other pollsters, who still give the Conservatives a more comfortable lead), it is hard to see how May could stay prime minister. She could be gone by Friday. Even if she sees the Conservatives through to a new term in government – as is still by far the most likely result in Thursday's general election – May could stagger back through the doors of Number 10 with her authority shattered. Knives will be sharpened. Jeremy Corbyn and Theresa May attend the 2016 Remembrance Sunday Service at the Cenotaph on Whitehall. Credit:MOD This might happen. Again, it should be emphasised it's still not the most likely outcome. But as of the past seven days, it is no longer the risible dream of a few ragged-trousered lefties, raving on social media about the bias of the MSM (mainstream media) against the jam-making, allotment-tending, friend-of-the-disadvantaged-but-not-his-fellow-MPs Jeremy Corbyn.

And the fingers of blame are all pointing in one direction: Number 10. In the last week, Theresa May has switched her campaign to a focus on Brexit and immigration. Credit:Getty Images Just a week before she called the election May appeared untouchable. She was the new improved Iron Lady, a "red Tory" who loved traditional values and believed the state should help the "just about managing". Almost half the country – including many 2015 Labour voters – chose her as the best prime minister; fewer than one in eight chose Corbyn. More than half of Britain thought she was doing well as PM and "has what it takes to get things done", according to an April YouGov poll. She was seen as decisive, honest, and good in a crisis. A tenet of the Labour party's campaign strategy has been to engage younger voters. Credit:PA

She reminded them of Margaret Thatcher more than any other prime minister since the 70s – and not just because of her gender (both were Oxford educated, hard-working and from strongly Christian families). There were warning signs. The YouGov survey also found her widely perceived as out of touch with the concerns of ordinary people – and "cold". Jeremy Corbyn took part in a televised leaders' debate with, among others, Liberal Democrats leader Tim Farron. Theresa May, who did not debate, suggested she would instead be focusing on Brexit negotiations. Credit:Getty Images And among those who chose her as the best prime minister, about half said it was because of Corbyn's weaknesses, not May's strengths. May's public persona was a confection: the assumptions of an electorate who had not yet come to know their new leader.

Some say it has been a risk for Theresa May to base her campaign on trust. Credit:Getty Images Matthew Goodwin, professor of politics at the University of Kent, says the public didn't really know Theresa May. "Now they're getting to know her and if you look at her performance ... she's still a way off from being a polished performer of the type either [former Labour leader Ed] Miliband or [former Conservative PM David] Cameron were. She's quite anxious, she's nervous." Labour supporters wait for Jeremy Corbyn at an event in Reading, which is in the same county as Theresa May's home constituency of Maidenhead. Credit:Getty Images In 2016 Labour's Yvette Cooper, who shadowed May as home secretary, wrote that May was "not fleet of foot when crises build, she digs in her heels. And she hides when things go wrong."

A prime minister cannot hide in a crisis. Nor in an election campaign going off the rails. May framed her campaign around issues of leadership and Brexit, telling voters that only she could be trusted to lead a "strong and stable" government to get the best exit deal from the EU. She spent the first weeks of the campaign crowbarring that phrase into interviews. Asked about wages, she said the "strong and stable leadership of the Conservatives" would boost the economy. On taxes, she said they'd come down through "strong and stable leadership". Asked what a "mugwump" was (long story), she replied: "What I recognise is that what we need in this country is strong and stable leadership." This robotic repetition is a technique beloved by the Conservatives' chief election strategist, Australia's own Sir Lynton Crosby, who says it works to "cut through the static" of the modern media. In 2015 David Cameron won with his ad nauseum catchphrase "long-term economic plan", which Sir Lynton later said was a simple statement about trustworthiness: "The emotional element was the risk to those jobs, the risk to that economic plan that Ed Miliband and his weakness would pose."

Sir Lynton said he was inspired by a 2007 book by an American psychologist called The Political Brain. In it, the author writes "the goal is to convince voters that your candidate is trustworthy, empathic, and capable of strong leadership, and to raise doubts about the opposition along one or more of these dimensions". You can almost imagine Sir Lynton thumbing through The Political Brain to this line before coming up with "strong and stable leadership". Robotic political catchphrases annoy the hell out of journalists. On Wednesday a Plymouth Herald reporter complained his interview with May had been "three minutes of nothing", like a game show in which she had been challenged to "talk without clarity, candour or transparency". On Thursday, a TV journalist asked the PM: "Isn't the reason that you are doing so badly that whenever people ask you about policy, all they get are cliches and platitudes ... People think there is nothing there?" Former UKIP leader Nigel Farage put it like this: "She looks insincere and really rather cold compared to Mr Corbyn who, despite his mistakes and appalling historical associations, appears to be having a ball."

But beyond aesthetics, it was a risk to base May's campaign on trust. May had started the election with a U-turn, belying her "strong and stable" image. She had repeatedly said there wouldn't be an early election, then suddenly there was. Her party manifesto included a plan to reduce social support for the elderly – quickly dubbed a "dementia tax". May eventually announced a cap on the costs. This was not looking "strong and stable". Party strategists held a crisis meeting and decreed a new focus on Brexit and immigration. In a speech on Thursday, May accused Corbyn of "not believing in Britain" and not believing in Brexit.

"You can only deliver Brexit if you believe in Brexit," May said. May campaigned for Remain in last year's referendum. "This has been a remarkably volatile campaign," says John Curtice, professor of politics at the University of Strathclyde and one of the country's leading election experts. "Usually in British [election] campaigns nothing happens. "Three weeks ago it basically looked as though the prime minister was almost bound to make it, now we don't know." However, he adds, "in one sense it's still a pretty boring election because it's still incredibly difficult for Jeremy Corbyn to actually displace the Conservatives as the largest party."

It is difficult to win a landslide majority in Britain, Curtice says, because Scotland is mostly in the hands of the Scottish National Party (SNP) and there aren't many marginal seats. He still projects a Conservative win, but the question is about how well they win, he says. "That is potentially politically crucial to the position inside the House of Commons on Brexit, to the future political authority of the prime minister, and perhaps the future internal politics of the Conservative party." So what happened? Labour has won back a lot of voters who were toying with voting Conservative – the ones who worried "if my dad knew I was thinking voting Conservative he'd turn in his grave", as Times columnist Danny Finkelstein put it.

Another big change has been a surge in the youth vote, Curtice says. In late April, 41 per cent of the 18-24s said they would vote Labour – that number is now 64 per cent. "Younger voters are less likely to vote, so what is now crucial is what is the level of turnout among young voters," Curtice says. "The Corbynistas' strategy was avowedly to persuade young people to vote Labour and to turn out. It looks as though they've achieved the first of those objectives, the crucial question now is whether they can achieve the second." Meanwhile May's support is grounded in voters who voted Leave in the Brexit referendum – and the over-65s love the Conservatives more than ever, from 49 per cent in 2015 to 65 per cent now – an age group that turned out in droves for the 2015 election and the 2016 referendum. Goodwin says if May comes through the election with a smaller-than-promised majority then she should expect a reaction from her party's Thatcherite wing, telling her "enough of this 'red Tory' nonsense". "Conservatives will think like this: that Number 10 has been the most insular, irritating, frustrating Downing Street they've had in their lifetime, where they've not had access to the Prime Minister, where most of her advisers have irritated MPs, where they've advocated a brand of Conservatism that they don't consider to be true Conservatism, where state intervention and regulation has flown in the face of everything they stood for in the 80s and 90s.