Chester by-election: not as good as it looks for Labour

If the swing in the Chester by-election were repeated throughout Britain, Labour would have a majority of around 50. In that sense, the party can claim to be on course for a comfortable victory at the next election.

“Repeated throughout Britain….” Those words, or something similar, are uttered by every opposition party after every mid-term by-election where it gains ground on the Government. However, history warns us to treat such claims with care. The following table records the big post-war by-election swings to Labour. The good news for Labour is that Chester is its best result in the past 12 years of Conservative-led governments.

The bad news is that, if the past is any guide, it’s not nearly big enough to produce a Labour majority at the next election. The table shows what Labour’s national lead would be if each swing were reflected throughout Britain. The Chester by-election implies a 15.8% Labour lead nationally – which does, indeed translate into a majority of around 50.

The problem with this extrapolation is that the “repeated throughout Britain” assumption has never applied in practice. The table compares the national result implied by each by-election with what actually happened at the following election. The final column shows the difference between these two figures.

In all fifteen “big swing” by-elections between 1958 and 2012, the difference between the implied national lead and what actually happened was at least 12 percentage points, and after  all but three of the by-elections the drop was more than twenty points.

For example, ten years ago, Labour gained Corby on a similar swing to this year’s two comparable by-elections, Wakefield and Chester. But in Corby, Labour’s baseline was a national deficit of 7.2% – the amount by which the Tories defeated Labour in 2010. Taking that into account, the Corby result translated into an implied national lead for Labour of 18.2%.

By that measure, Labour fell short of its Corby performance last night. This time, the baseline was a nationwide Labour deficit of 11.8% at the last general election. This meant that a 13.8% swing in Chester translated into an implied national lead od 15.8%. And here’s the thing: after Corby, Labour went on to lose the following election. If Labour’s post-Corby history were to repeat itself precisely, then the Tories would win the next general election with a comfortable, and only slightly reduced, majority.

In other words, for Labour to secure a majority at the next election it must massively outperform its past record of translating mid-term by-election victories into its support at the following election.

Can Labour do that? Possibly. History does not always repeat itself. Each year sees a new edition of the Guinness Book of Records precisely because new records keep on being set. However, if I were a Labour strategist I would be more nervous today in private than the party claims to be in public – and if I were a leading Tory I would not be quite as downcast as the headlines suggest I should be.,