So what is happening?

The shrinking Tory lead is not the result of any slump in support for the Conservatives. Their support is holding up in the low to mid 40s. Rather the narrowing is due to Labour gaining support from the minor parties, who have struggled to cut through and who are losing ground. That suggests a polarisation of politics as those who are anti-Tory gravitate towards Labour.

They may have been encouraged to do so by the early phase of the campaign and specifically by contrasting manifestos. Although Labour’s launch was beset by leaks and confusion over costings, many of the flagship policies it contained – such as nationalising the railways and increasing spending schools and hospitals – are individually popular, especially with those inclined to support progressive parties. Whatever the motives behind the leak of the full manifesto ahead of its agreement and publication, that episode did at least secure greater coverage of the party’s programme than might have been the case. Labour has run quite a successful core vote strategy aimed at hanging on to 2015 voters rather than persuading switchers.

The Conservative manifesto was by contrast a very tightly controlled affair. But that tightness was perhaps responsible for the inclusion of a flagship policy on social care that quickly unravelled. That episode clearly tarnished the Prime Minister, who was closely associated with the policy, and YouGov recorded a significant temporary slump in her personal ratings following the U-turn.

While she has since partially recovered from that dip, a feature of the campaign is the Prime Minister’s difficulty in finding the right tone with voters. She makes few mistakes in interviews but she is yet to inspire.

Could Labour gain further ground?

While the recent trend has clearly been encouraging for Labour, and Tory spinners will warn that ‘everything is to play for’, it would be premature to conclude that the party might hold or even improve its current parliamentary position. As various commentators have noted, Labour frequently enjoys inflated poll ratings during the campaign which fail to be realised on election day. Indeed, in five elections since 1979 when the Tories have been in government, the polls have on average over-stated Labour’s share of the vote.

That may reflect the fact that at this stage of the game voters are responding with their hearts and not their heads. When faced with the actual decision of who they want running the country, different calculations come into effect. These are generally about who would make the best Prime Minister and who would deliver economic success. In this election, two other factors may be added: First, who is best placed to deliver Brexit? Second, following the tragic events in Manchester this week, who is most trusted with defence and security?

It is too early yet to understand the impact that the terrorist attack will have on the election. Jeremy Corbyn’s decision to make a controversial speech today linking the Manchester bombing to UK foreign policy will be seized upon by Tory strategists who are keen to focus on the current Labour leadership’s links with terrorist organisations. However, Corbyn’s stance may actually appeal to working class UKIP voters who blame foreign wars for Muslim immigration.

But in general terms what recent events undoubtedly do is make the question of leadership more prominent. And on that, and indeed on all of the key measures noted above, YouGov finds Theresa May and the Conservatives hold considerable leads, which could prove critical on polling day.