If the Conservative Party’s defining split is over Europe, Labour’s enduring divide is about the balance between purity and power. There is an underlying anxiety on the left that the compromises required to win are a betrayal of the true socialist cause.

The party’s fractious relationship with Tony Blair has always been about more than Iraq: Labour’s most electorally successful leader is loathed by some precisely because he won three elections, not despite taking the party into power. A certain section of the left has always been more comfortable machinating to seize control of the party than trying to change the country.

There are growing tensions between Jeremy Corbyn and John McDonnell but they are not about ambition or ideology, they are about political purpose.