CAMBRIDGE, England — Jeremy Corbyn remains an outsider in the battle for No. 10. But when it comes to the election campaign, there's no doubt it's the Labour leader who is defying expectations and Theresa May who is on the rack.

Under Corbyn's stewardship Labour has seen the biggest increase in poll ratings for one of the two main parties in any general election campaign in history, jumping from less than 30 percent to almost 40 percent according to some pollsters who assume a large turnout among young voters.

"If it transpires in the ballot box it will be the biggest increase that any of the two largest parties has managed to achieve in an election," said Professor John Curtice, Britain's pre-eminent pollster.

Though Labour started from a low base, with less than a week to go Corbyn is within touching distance of depriving May of a majority in the House of Commons, throwing British politics and the government's Brexit negotiations into chaos.

"It is not a massive surprise he is doing better on the campaign trail than he does in Westminster" — senior Tory campaign official

POLITICO followed the Labour leader on the campaign trail this week to understand what was behind his surge: joining hundreds of adoring fans at a rally in Reading Wednesday morning; witnessing him deliver his surprise announcement that he would take part in the BBC's seven-way debate; and then spending the evening in the Cambridge spin room where senior party figures capitalized on the prime minister's no-show.

Visits to other marginal constituencies, including in Greater Manchester, and conversations with Labour candidates in target seats over the past few weeks have also revealed that those once fearing a Labour wipe-out are now more confident they can hold onto their seats come June 8.

Here are the eight ways Corbyn is winning the campaign.

1. Labour discipline

The Labour Party has run a traditional left-wing "giveaway" campaign. Tory campaign insiders, who spoke on condition of anonymity, have been impressed with its "discipline."

The party's manifesto launch, offering a clear alternative to continued Tory spending cuts with populist offers on tuition fees and free school meals, has tempted a significant group of swing voters.

May has performed poorly. Hesitant, wooden and inflexible, the prime minister has proved a weak campaigner.

"There's a reason parties go with these giveaway campaigns — it's because they work," one senior Tory campaign official said.

Conservative MP James Cleverly said: "The tightening in the poll is predominantly down to Labour doing significantly better at this point in the campaign than they did at the beginning of the campaign."

"This is what Corbyn enjoys doing. This is what he is good at. He is doing stump speeches and a rousing call to arms. This is what won him the Labour leadership twice. He is in his natural turf. It is not a massive surprise he is doing better on the campaign trail than he does in Westminster."

2. Theresa May

May has performed poorly. Hesitant, wooden and inflexible, the prime minister has proved a weak campaigner.

The theory among Tory MPs (and Labour "moderates" opposed to Corbyn) was the more the public saw of Corbyn the more they would dislike him. If anything, it is May who has proved a turnoff. With just a week to go, her lead over Corbyn when pollsters asked people who would make the best prime minister has shrunk to 13 percent, down 4 percentage points in a week and fueled by a surge in support for Labour that's taken the party to within 3 points of the Tories in one poll.

Many commentators are pointing to parallels with Donald Trump and Hillary Clinton in the U.S., but it is perhaps closer to the the Democratic primary race in which Bernie Sanders, the insurgent outsider, ran the solid but unlikeable frontrunner far closer than should ever have been the case.

May, however, does not have any super delegates to help her.

3. The airwaves war

As the campaign has gone on, Corbyn has become increasingly assured in front of the media.

After surviving a grilling from famously tough political interviewer Jeremy Paxman, Corbyn breezed through a sofa chat on prime time TV hit The One Show and won plaudits for simply turning up to the seven-way leaders' debate.

Corbyn may not have landed any big blows Wednesday night, but he avoided major gaffes and won a raft of headlines after putting the absent prime minister on the spot.

Wes Streeting, a Labour MP who has been openly critical of Corbyn in the past, and who is fighting to retain his very marginal Ilford North seat, said: “Jeremy has certainly surpassed expectations during the course of the general election and done better in TV interviews and debates than people may have expected. By contrast I think Theresa May has really taken voters for granted."

In the TV debate spin room, senior Labour figures piled in. Emily Thornberry, the shadow foreign secretary, told POLITICO: “Theresa May should have come. I genuinely don’t understand how she thinks she can get away with hiding in the way that she is."

4. Accepting Brexit

For the Liberal Democrats this was meant to be the comeback election. The party of the 48 percent of Remain voters who would flock to its leader Tim Farron to stop Brexit in a second referendum. Polls, however, suggest a return to the big time is further away than ever.

"The whole thing about having a second referendum is just not working for the Lib Dems at all," one Labour candidate fearful of not regaining his seat a month ago but now "feeling good," told POLITICO.

"They are having an absolute shocker. People just don't want to rerun the whole thing. They think that decision was taken a year ago and want to talk about other things — schools, hospitals and all of that."

Expectations have also played a part in Corbyn’s success and will likely shape how the result of the election is viewed.

Corbyn's decision to order Labour MPs to vote for the triggering of Article 50 left the party free to focus on other issues during this election campaign instead of refighting the referendum campaign like the Lib Dems, the Labour candidate said. "In hindsight, it would've been disastrous for us. You have to hand that to Corbyn," said the fierce critic.

Clive Lewis, who resigned from the shadow cabinet over Labour's position on Brexit, and has openly supported the idea of a second referendum, said Brexit rarely came up on the doorstep in his Remain-voting constituency. "They [the Liberal Democrats] bet their house on a Brexit election and it didn’t materialize."

5. Low expectations

Expectations have also played a part in Corbyn's success and will likely shape how the result of the election is viewed.

Lewis said success for him would be a win, but added: "Considering where we were, considering the media onslaught we faced and the division within our own party before the election started, considering the issue of Brexit ... we will wait and see on the night what it comes to, but people are talking about a hung parliament."

He said that would exceed the expectations of most in the party. "Success is relative to where we were. Where we were heading at the beginning of this election was being talked up as a 200-seat majority for the Tories, we were looking at being wiped out."

6. Tory manifesto madness

All candidates need a bit of luck and it has been as much the May team's mistakes as Labour's appeal which has helped to boost Corbyn's support. All candidates point to the gift May's team handed Labour in the form of her manifesto pledges on care for the elderly.

The decision to help plug a funding black hole with the assets worth over £100,000 of those who need social care — recouped after they had died — was swiftly dubbed a "dementia tax" and has not gone down well on the doorstep.

Streeting said: "The turning point locally has been around social care and pensioners. I think people felt quite angry at the policy she put out, particularly because it was so half-baked and the double whammy of ending the triple lock [which protects pensions], going after the money in people's homes and savings to pay for social care and cutting the winter fuel allowance has made lots of pensioners really angry."

But it was not just the policy which lost May support — her subsequent announcement of a cap on the amount people would pay and perception of a U-turn also lost her support.

In Bury, a former mill town in Greater Manchester last week, 75-year-old Theresa Short told POLITICO May's decision to announce a cap had shown her to be "changeable."

"While she is saying 'strong and stable,' they are only words," Short added. Her opinion of Corbyn had changed through the campaign as she had heard more of him, she said. "He seems to have good ideas. I heard him on the radio the other day. I was astonished."

7. The youth vote

Many of those who turned up at the rally in Reading were young people — and that has been mirrored across the country.

But the big question for the Labour Party is whether they will actually turn out to vote. Previous poll boosts believed to be from young voters — including "Cleggmania" for Nick Clegg and "Milifandom" for Ed Miliband — failed to materialize on election day. And in the last general election, 18-to-24-year-olds were almost half as likely to vote as those aged over 65 (43 percent vs 78 percent).

Eleanor Jeffery, a 19-year-old from Basingstoke, who was at the Reading event, said: "Ever since [the referendum] young people have realized they need to get involved. They thought we were going to stay in [the European Union] and unfortunately [Brexit] happened and people realized they needed to vote more and take charge of their country rather than sit back and watch it happen."

Her friend Archie James, 18, an enthusiastic Labour supporter who was not a Corbyn fan, said he did not think Theresa May would get the result she was expecting. “Despite people not liking Jeremy Corbyn, people we know who are apolitical ... these policies resonate with them. People I would not expect to be voting are voting and they are voting Labour," he said.

8. Standing room only

While packed rallies are not an indicator of voting intentions, Corbyn has managed to attract people to his campaign events in their thousands, giving him a sense of momentum when the images are beamed into people's living rooms on the evening news. "I know the rallies are driving them mad at CCHQ [Tory headquarters]," one former Tory campaign strategist said. "They know they can't match it and it's a problem for them. The images are really good for Labour."

On a Wednesday morning in Reading, what appeared to be about 1,000 people turned up to the car park of a leisure center on the outskirts of town at short notice. A senior Labour campaign figure said the scale of the spontaneous events was big and it was growing, and Corbyn is expected to hold more in the final days of the campaign.

Thornberry dismissed the importance of the polls, telling POLITICO they did not help one way or another. "What helps us is the reaction we get when we are talking to people, because we do have this advantage — our 600,000 members are out there talking to people, so we are sensing a turn. We are sensing a change, we are sensing people are now starting to think this is a choice," she added.