When Theresa May called her “snap election” seven weeks ago, the polls were showing Labour facing a general election defeat on the scale of Michael Foot’s 1983 disaster when the party was reduced to just 209 MPs.

Her newspaper cheerleaders, including the Sun, were declaring her the “new Maggie” and claiming she had the opportunity to kill off Labour and end its days as a serious electoral force. But, as ICM’s preliminary call on their final poll showed, May goes into election day with a 12-point lead of 46% to Labour’s 34%. This is up from David Cameron’s seven-point victory just two years ago and represents a swing to the Conservatives of 2.5%.

The ICM results pointed to a Tory majority of 96 on electoral calculus estimates and it is at the higher end of seat predictions, but still short of the landslide many pundits were predicting six weeks ago. “This final poll confirms the pattern that ICM has produced over the last fortnight: a fairly healthy and static Conservative share with consolidation of the Labour bump first witnessed after the manifesto publication,” said ICM’s Martin Boon.

The detail of the Guardian/ICM poll confirms Corbyn’s popularity among younger voters. He enjoys a lead of 66-23 among 18 to 24 year olds and 47-33 among 25 to 33 year olds, but the declared intended turnout of both these groups, at 64% and 70%, is 10 points below other age groups.

Perhaps more importantly, the Conservatives seem to have won the battle of the working class, with an even larger 23-point lead for May among the key skilled working-class C2 voters often found in many marginal swing seats across the Midlands. However, Labour seemed to have halted the Tory advance among unskilled DE voters, where it has regained a modest two-point 38-36 lead.

More disappointing for Labour is that the Tories enjoy a slim one-point lead in the marginal seats it is defending, 45% to 44%.

As the Guardian poll-tracker confirms, the polls have narrowed. Two terror attacks have scarred a tumultuous campaign in which Jeremy Corbyn’s personal ratings rose from a very low base and May’s wooden leadership style dispelled the appetite for Thatcher comparisons.

That volatile campaign has had an impact on the parties’ standings. May and Corbyn go into election day knowing that a landslide Tory victory is now unlikely. Nevertheless, insiders in both parties concede that a healthy 50-plus Conservative majority is the most likely outcome.

In the meantime, the minor parties have been sidelined, with the collapse of Ukip in particular fuelling the expected scale of the Conservative victory.

But as the Southampton University-based Polling Observatory pointed out, the variation in the size of the lead among the pollsters, from 10 to 12 points on ICM and Comres to 2 to 4 points on Yougov and Survation, is almost wholly down to the different adjustments being applied for turnout, particularly among younger and poorer voters.

The final Times/YouGov poll had the lead widening to 7 points, with the Conservatives on 42% and Labour on 35%, implying a Tory majority of 50 plus.

The preliminary figures for the last ICM poll of the campaign showed that the actual movement in the campaign has been limited. The Tories have gone down from a 48% share of the vote on 24 April to 46% on the eve of the election, while Labour has risen seven points, from 27% to 34%.

These figures imply an electoral calculus majority in the 90s for May and Labour losing up to 33 of its current 229 seats, leaving it worse off than Foot’s 1983 total of 209, despite a higher share of the vote. The Conservatives could gain up to 42 seats, adding to their current total of 330.

Most of that gain has come from the minor parties, with the Liberal Democrats down 3 points to 7% in the latest poll, Ukip down two to 5% and the Greens down one point to 2%. ICM interviews were continuing through Wednesday before final figures were released.

By contrast, YouGov’s poll published on Wednesday – which does not adjust for lower historic turnouts for younger and poorer voters – put Labour just four points behind on 38% to the Tories’ 42%, which would leave May 24 seats short of an overall majority in a hung parliament.

As for expectations, the ICM poll showed that only one in 10 (12%) of voters expects a Tory majority at the top end of the range of predictions, with a plurality (38%) believing May’s majority will be secured, but only by double figures. Fewer than one in five (17%) expect a hung parliament, with the great optimists being the 7% who think Labour will secure the keys to Downing Street (18% of Labour voters believed that Jeremy Corbyn would smash it).

Rob Ford and Will Jennings of the Polling Observatory said the upturn in Labour support during the campaign has been on a similar scale to the Lib Dem “Cleggmania” surge in 2010. Clegg’s rise in popularity was concentrated in just six days after his appearance in the televised leaders’ debate and had largely dissipated by the time people got to the polling station. In contrast, Labour rise this time has been steady over a six-week period, although there are concerns that it has largely piled up votes in seats the party already holds. The hopes and aspirations of party strategists are most clearly revealed by the leaders’ final day itineraries. May spent hers touring Southampton, Norwich, Nottinghamshire and the West Midlands before a final rally in Birmingham – all rich with Labour marginal seats up to number 50 on their hit list. Corbyn spent his day in Glasgow, Halton in Cheshire, Colwny Bay, Watford and Harrow, before a final rally in London. Corbyn took in a mixture of seats: some safe Labour, some safe Tory, with one or two marginals that the party is defending among them. But at nearly all he was greeted by an enthusiastic crowd of supporters.

Tim Farron was scheduled to spend his final day in Lib Dem targets including Solihull, St Albans, Twickenham (their top targe), Bath and Oxford, as well as Tom Brake’s seat in Carshalton, where a local council bin crisis has hit his chances of holding his seat.

Election polls tracker 2017: Survation has Labour almost level with Tories Read more

The election has proved as much a test for the pollsters as the politicians. The wide spread in Tory leads underpinned by a different approach in methods means that some polling companies will face acute embarrassment on Friday morning. Whatever the result, public confidence is unlikely to rise in a polling industry that has had more than its fair share of misses in recent years.

ICM Unlimited interviewed a representative online sample of 1,532 British adults aged 18+ on 6-7 June 2017. Interviews were conducted across the country and the results have been weighted to the profile of all adults. ICM is a member of the British Polling Council and abides by its rules.