Quite long and data-y. Key points:

Historical polls show the gap between satisfaction with Labour and Tory leaders, in polls close to elections, is an extremely good predictor of the gap between the parties at the election.

It is actually a better predictor of the gap between the parties than voting intention data.

The measure suggests the Tories are on course for a 15-18pt win.

This means current polls may be overstating the Tories’ lead.

Two years ago this week, Matt Singh correctly argued, against the consensus, that the Tories were on course for a clear win.

This post recreates one of the analyses Matt used to come to his conclusion. Using it, I suggest the Tories are on course for a 15-18pt victory.

I’m using Mori’s long-running data on satisfaction with the party leaders. This was one of several that Matt used and is being widely talked about as an apparent sign that the Tories will win (note, Matt used several other measures as well – my analysis is less thorough than his and I’m not suggesting this is such a serious study as his was).

Some high-profile commentators treat this satisfaction data as if it’s a direct substitute for voting intent. So when, for example, 23% say they are satisfied with the job Corbyn was doing, some people have interpreted that as meaning Labour would get 23% of the vote. That is not borne out by the data.

The following charts compare satisfaction with the leaders, from Mori’s polls taken six weeks before the election (chosen so we can compare with the latest Mori poll, from April 2017).

The first shows the percentage satisfied with the Prime Minister minus the percentage satisfied with the Leader of the Opposition, compared with the governing party’s lead at the election.

The second is essentially the same, but shows the difference in net satisfaction between the leaders, so also takes into account how many are dissatisfied.

For each I’ve used a polynomial regression, as Matt did. This points towards a Tory victory of 15pts if we compare only those who are satisfied. If we compare net satisfaction the Tory victory is 17pts. Both have nearly equally good r-squared values. The fit is almost exactly as good with a linear regression but in that case the Tory victory is around 1pt greater. So I suggest this points to a Tory victory of 15-18 points.

(This is in GB only, but that shouldn’t make much difference to the gap between the parties expressed in terms of UK results.)

This suggests the Tories are likely to exceed their 1983 victory, but probably not 1931.

It would mean current polls are mostly overstating the Tories’ lead. The last five have given the Tories a lead of 16-22pts, which appears a little high.

That said, it may be that opinion has shifted since the latest Mori poll. Perhaps the unusually short period between elections means opinion is less settled than it normally is before an election, so the 6-week-out poll isn’t as accurate as it normally is. The next Mori poll, out in a couple of weeks, will tell us whether satisfaction with the leaders is indeed still shifting.

On a final note: I was wrong. I’ve said a few times that I think the suggestion that analysts should look beyond voting intention numbers, to underlying figures, was like astrology – you could always find something in the underlying numbers to, retrospectively, fit with what actually happened.

But this analysis has changed my mind (perhaps it should have been changed by Matt’s analysis two years ago). I’ve also run the same dependent variable – election results – against the voting intention data in the same Mori polls that I took the leadership-satisfaction scores from. Unexpectedly, to me at least, the correlation between voting intent (in terms of gap between the parties) and the election result was worse than the correlation between leadership satisfaction and the result (the same is still true of the final Mori poll before the election, although the difference is smaller):

In short, if you want to project the gap between the parties at the election, it’s better to look at the leadership satisfaction – and apply the regression formula – than it is to look at voting intent.

Unless perceptions of Corbyn and May shift dramatically, the Tories are heading for the biggest win since before the Second World War.