Jeremy Corbyn is currently the frontrunner for the leadership of the British Labour Party. While few would have bet on him when he announced his bid, he is now the bookies’ favourite.

A number of prominent figure on the right of the party have come forward to warn against the electoral consequences of him becoming leader. Tony Blair wrote that Labour would face “annihilation” if the left-winger were to lead the party, and Gordon Brown explained at great length how a Corbyn leadership would affect the party’s credibility. The general theory among pundits is that only a centrist candidate can be able to win elections in Britain. There are three reasons behind this. The first is institutional: in a majoritarian system, you win elections in the centre, and you need to convince the voter in the middle of the voter distribution. The second and third are historical: Tony Blair managed to win three consecutive elections on a centrist (or centre-right) platform; and Michael Foot faced a mortifying defeat in 1983 with a clear left-wing platform. But how does the electoral score relate to the left-right position of parties?

In the Graph 2 below (click for interactive version), I have plotted the vote share of the Labour and Conservative parties between 1945 and 2010 against their left-right position as measured by the comparative manifesto project. In other posts I have used the Chapel Hill Expert Survey, but the manifesto data goes back much further in time. Regression is faced with a number of problems: endogeneity (it is difficult to know whether the ideological position determines success, or whether electoral success in previous elections conditions the electoral position), measurement problems (the British party system has become more multipartisan over the last 30 years, so vote shares have declined for both parties). The correlation is very weak, and the relationship is not significant. The regression line (you can see it if you click on the graph) slopes downwards for both parties, but it has probably more to do with the fact that both parties were more left-wing in the 1950s, and also gathered more votes, than being more left-wing yielding more votes.

However, the representation of the data gives an interesting representation of the history of British politics since 1945. In the postwar period, both major parties were on the left according to the data. That’s the period in which the NHS was created and major industries were nationalised. Throughout the 1950s and 1960s, the difference between the two parties was not that big (Graph 3). This changed to a great extent in the 1970s, when the distance between Labour and Tories increased and a major period of polarisation started. The Thatcher era is of course the most symptomatic of this: the Tories veered to the right, while Labour engaged in soul-searching with large swings up and down (Graph 3). The Blair leadership shifted the party to the right, bringing back electoral success. The distance between the two main parties decreased again, but this time, convergence took place on the right rather than on the left, as in the 1950s and 1960s.

In graph 2, it is striking to see two parallel lines going right and upwards: the Tories between 1974 and 1979 (Thatcher) and Labour between 1992 and 1997 (Blair). In both cases, a major shift to the right coincided with a major increase in vote share (following crushing defeats). In contrast, the two instances where the Labour party veered to the left (in 1974 and 1983) coincided with lower vote shares. A move to the left did coincide with a higher vote share between 1987 and 1992, but this was not enough to secure a majority in parliament.