Yet as voting day looms, those fears are looking less and less likely. Instead, the Conservatives are starting to get worried.
Take a look at the average of polls collated by groups like Britain Elects and you'll see a clear surge in support for Labour. If the polls are right, this means that May could actually lose seats in Parliament rather than gain them. At least one poll has raised the possibility of a hung parliament, where no one party would have an outright majority — a scenario that could even see Corbyn as prime minister, if he could cobble together a coalition.
This unexpected scenario has prompted two big questions to hang over British politics for the past week or so. First, what could have happened to make Britons rethink their opinion of Corbyn, previously derided as “unelectable” by his critics? And secondly, could the polls just be wrong?
When May announced the snap election in mid-April, analysts suggested that she was seeking a mandate to personally lead the country through its exit from the European Union. The prime minister, who had campaigned against Brexit, had come to lead Britain after Prime Minister David Cameron stepped down after last year's referendum. This would be her first major test with national voters.
Though she had previously suggested she would not call a snap vote, May seems to have looked at Labour's low polling numbers and thought that now was an opportune time to call an election. It's an understandable impulse: A big win would not only give her a larger majority in Parliament, but it also potentially would give her another five years until the next election — during which time she could focus on the complicated business of leaving the E.U.
But things weren't so simple. While the prime minister attempted a “presidential"-style campaign that focused on her “strong and stable leadership” slogan, May often cut an awkward figure at campaign events and faced a backlash for her refusal to take part in debates with other parties' candidates. Her attempts to stay on message, no matter what, led to criticism that her campaign lacked any substance.
“Is the reason that you’re doing so badly is that whenever people ask you about policy, all we get are cliches and platitudes?” Channel 4's Michael Crick asked May during a tense question-and-answer session with journalists this week.
There were problems with the substance, too. When the Conservatives announced a plan to make elderly people pay for care in their own home if they had more than £100,000 ($128,000) in assets — a move seen as potentially forcing some to sell their home to pay for treatment — the backlash was swift. Labour swiftly dubbed the policy a “dementia tax,” and the Conservatives were forced to make a U-turn on the policy. Some news outlets argued that the controversy had cost May 5 percentage points off her lead.
At the same time, Corbyn has been able to portray himself as an anti-establishment underdog, proffering populist spending increases, and he has largely avoided major mistakes. Labour is buoyed not only by a dedicated young and social media savvy fan base that is often able to dictate the terms of the debate, but also the lackluster performance of Britain's other major parties, the Liberal Democrats and UKIP.
The pollster's predicament
There is a catch, however. Britain's polls have a history of overestimating Labour support — most recently in 2015, when most expected Corbyn's predecessor, Ed Miliband, to win election and become prime minister. He didn't. Instead, his loss led to an identity crisis and leadership contest that resulted in Corbyn, a member of the party's once-isolated left wing, ending up as an election candidate.
Polling companies went back to the drawing board after 2015, hoping to understand where they went wrong. Most ended up agreeing that they had paid too much attention to enthusiastic young voters, ignoring the fact that these young voters often don't actually vote (both the 2015 general election and the Brexit referendum saw 18- to 24-year-olds less likely to vote than those over 65).
The polling companies most optimistic about Corbyn's chances seem to have placed their faith in these young voters again this year, taking them at their word that they will vote and that they will generally do so for Corbyn. Anthony Wells, director of the political and social research team works in YouGov's London office, conceded this in a blog post published Thursday.
“From the pollsters’ point of view this is an experimental election,” Wells wrote. “We all got it wrong in 2015 and we are all trying different methods to get it right this year.”
Notably, it was YouGov that made perhaps the most pro-Corbyn prediction this week, estimating that the Conservatives would lose 20 seats and with it their overall majority. The poll caught widespread criticism from politicos and polling pros alike, with some noting that Britain's first-past-the-post system (whereby voters vote for a local politician, not a nationwide leader) made seat predictions an exceptionally risky game. Under this system, it's a very really possibility that Labour, for example, could have more votes than it did in 2015 but actually lose seats.
But YouGov is hoping that its unorthodox polling method — which uses an unusually large sample size and a complicated model to map the results to the entire country — could prove more accurate than its rivals. It's a big gamble. After 2015 and last year's Brexit vote, a lot of people will be watching to see whether Britain's pollsters can get it right, this time.