What a difference two percentage points makes. YouGov’s second and much-hyped constituency-by-constituency poll puts the Conservatives unchanged on 43% and Labour on 34%, up two points on its previous effort a fortnight before.

The result is that Boris Johnson’s notional majority has been cut from 68 to 28 as his party’s seat count falls by 20 to 339 and Labour’s improves by the same amount to 231. The SNP takes 41 and the Liberal Democrats 15.

All this reflects how sensitive the British political system is to slight variations in voting patterns, which is why the Conservatives are so nervous about how the party leader responds to a story about a young child having to sleep on a hospital floor. After all, while the Tories remain favourites, a hung parliament cannot be ruled out.

The method used, the so-called MRP (which stands for a modelling technique called multilevel regression and post-stratification), models a result for each constituency by creating a profile for how various demographic groups might vote, based on a large sample of 100,000 interviews conducted over the previous six days.

First time around, the MRP was conducted after the Conservatives had largely squeezed out the Brexit party, but before Labour staged a modest recovery, when it, in turn, began to squeeze out the Lib Dems. The second poll shows precisely the impact that mini revival has had.

This time Labour is predicted to make two gains, both in London, in Putney and Chipping Barnet, although still not any of the seats where the party has had a strong ground game with activists pounding the seats: Southampton Itchen, Hastings and Rye and Chingford and Woodford Green.

Tactical voting across the UK: a region-by-region guide Read more

As for the Conservatives, the poll predicts gains largely from Labour in the Midlands and north of England, including Sedgefield in County Durham (once held by Tony Blair), Bolsover, where Dennis Skinner is running again, as well as seats like Barrow-in-Furness and Ashfield in Nottinghamshire, where Labour insiders are already conceding the cause is all but lost.

But it is the Lib Dem figures that are particularly eye opening. YouGov thinks the party could overturn a 9,999 majority in Winchester, pick up the South Cambridgeshire seat where Heidi Allen was briefly an MP for the party – and even predicts the party is in touching distance of foreign secretary Dominic Raab in Esher and Walton, where the majority was 23,298.

If that proved to be the case, one might expect more dramatic results elsewhere. It suggests that the traditional Conservative vote is more vulnerable in the commuter belt than previously thought. But that is also seductive speculation: the poll’s principal conclusion is that Johnson is heading for a close win.

With a day to go until polls open on Thursday, two important stories have become clear. Will traditional Labour voters continue to turn their backs on the party led by Jeremy Corbyn? Could there be a late movement away from the Tories in London and the south east? Either way, it sets up a nerve-jangling finish.

• This article was amended on 11 December 2019. An earlier version misnamed the constituency of Chingford and Woodford Green as “Chingford and Wood Green”, and misplaced Ashfield as being in Derbyshire.