One of the paradoxes of last year's general election is that the Tories had to increase their vote sharply just to avoid going backwards any further than they did. Theresa May took 42% of the popular vote and yet lost the overall majority that had been won by David Cameron two years earlier with only 37% of the vote. It's easy to dismiss what happened as a form of general polarisation in which both the Conservatives and Labour were bound to see their support increase while smaller parties were inevitably squeezed out, but in fact the processes that led to the Tory and Labour increases were largely separate. UKIP voters went home to the Tories because the issue of Brexit seemed to be settled (laughable in retrospect, I know), while Labour were only able to capture former abstainers and Green voters because Corbyn had become leader - something that had absolutely nothing to do with Brexit. So if circumstances had been different it would have been perfectly possible for the Corbyn surge to occur without any corresponding swing back from UKIP to Tory - and we're now starting to see what the effects of that would have looked like.
As you probably know, for several months in the early part of this year, the Tories had re-established a small but significant GB-wide lead over Labour, but that was reversed at the time of the Chequers "deal"/shambles. Labour briefly went into the lead, but we now seem to be back to roughly a neck-and-neck race. Although Labour may have taken some support direct from the Tories, the most important impact of Chequers appears to have been to bring UKIP back from the dead. In every poll published in May and June, UKIP had been somewhere between 2% and 4%, and in most cases they were on 3%. Since Chequers, they've been hovering between 5% and 8%, with the most common figure being 6%. So their support has essentially doubled, and needless to say a lot of the extra votes are coming from the Tories. In the last two YouGov polls, 9% or 10% of respondents who voted Tory in 2017 said they would now vote UKIP, which compares to an equivalent figure of just 3% in the last YouGov poll of June. Labour's position relative to the Tories could therefore have improved without any direct boost for Labour at all (and indeed after the reversal of a temporary bounce that is effectively what has happened).
The question that forms in my mind is whether what we're currently seeing is merely a staging-post. UKIP's support may be double what it was a few weeks ago, but it's still only half of what it was at the 2015 election. With talk of Nigel Farage just possibly returning as leader next year, there's surely scope for a much bigger swing back from Tory to UKIP if the narrative of "Brexit betrayed" is allowed to develop. There's no particular reason to think Labour would lose support to smaller parties at the same time, which means that the polls could move firmly into Labour overall majority territory by default. Fear of that happening could be another constraint on Theresa May that will make it less likely that she'll agree to any deal remotely acceptable to the EU - thus further increasing the chances of a disastrous no deal Brexit.
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