When Theresa May announced the snap election in mid-April, most expected a dull campaign and a foregone conclusion. The prime minister had strong approval ratings and the Conservative party had a huge lead over the Labour opposition. The election looked like an easy way for the Tories to lock in their advantages over a weak opponent.

We should have known better. As the great political philosopher and boxer “Iron” Mike Tyson once observed: “Everyone has a plan until they get punched in the mouth.”

Labour support has surged in the polls, rising by around eight points in the polling average. The Conservatives have held on to most of their own earlier poll surge, achieved mainly through the mass defection of 2015 Ukip voters, so they remain ahead, but the gap has closed sharply. A lead of over 20 points has halved to under 10, with a lot of disagreement between pollsters. Some polls and projections now suggest May could even end up losing ground this week.

The smaller parties have been left behind as two-party politics returns with a vengeance. The hopes of the Liberal Democrats for a “Remain surge” have turned to dust, and the party now has a fight on its hands just to hold on to its handful of MPs. Ukip support has collapsed and is on the brink of extinction. Support for the Green party has halved. Voters have not consolidated behind the big two like this for more than a generation.

Such big shifts during campaigns are rare, although not unheard of – the 2010 election campaign, for example, also produced a large swing in the polls but, on that occasion, the projections pointing to a surge in Lib Dem support proved to be mistaken, as they were in 2015 when the campaign polling was more stable.

Will the current Labour surge be a third miss in a row for campaign polling, or are the pollsters – who all made changes to their methods after the 2015 miss – picking up a real shift?

A case can be made either way. Believers in the Labour surge can point to the broader polling evidence – it is not just Labour’s headline support that has improved but Jeremy Corbyn’s leadership ratings have also risen sharply, albeit from a low base, while May’s are down. Labour’s ratings on the issues have also improved, while pollsters suggest that the Tories’ very unpopular “dementia tax” social care proposal is the main policy to have “cut through” with voters during the campaign.

Surge sceptics, however, can also build a convincing case. Labour’s rise in the polls has been concentrated in demographics – in particular, younger voters and former non-voters – who tend to turn out at low levels but are now apparently very keen to vote. The Tories continue to win the other side of the generation game, with a huge and stable lead among pensioners.

Judgments of the Labour surge in the polls – and prospects on 8 June – hinge very strongly on whether the traditional “grey advantage” in turnout is reduced this time. There are two reasons to think it won’t be: the historical record and the record of the polls. Large generation gaps in turnout have been recorded in all the recent British elections, including the very high turnout for the EU and Scottish independence referendums. The polls also have longstanding problems with political engagement – they recruit too many political enthusiasts, and people taking polls tend to exaggerate their likelihood of voting. Pollsters also have a longstanding (and most likely related) tendency to overstate Labour support and understate Tory support – a pattern that has held in every election but one since 1987.

Labour surge sceptics can also draw different conclusions from a deep dive into the polling. While Corbyn has closed the gap on May in the leadership ratings, he still remains a long way behind. Much of Labour’s support still comes from voters who say they back the party but not its leader. Tory voters, by contrast, are rarely conflicted in this way.

Labour’s position on the issues has improved, but the Conservatives retain commanding leads on many of the central issues at stake, such as Brexit, economic management and immigration. Since the “dementia tax” U-turn, events have shifted the campaign agenda on to security and foreign policy, traditionally stronger terrain for the Tories.

The Conservatives remain strong favourites because they still hold the better hand – they retain a poll lead and strong support is from the most reliable voters in the electorate. Labour will require unusual voters to turn out in unlikely numbers. But, as any American knows, such upsets can happen in politics.

Robert Ford is professor of political science at the University of Manchester and the author, with Matthew Goodwin, of Revolt on the Right: Explaining Support for the Radical Right in Britain