The formula for stable government, according to Britain’s constitution, is a big parliamentary majority and divided opposition. Sir Keir Starmer’s predicament proves that those conditions are not sufficient.

The prime minister’s inability to convince voters that he has an agenda for national renewal, and the demoralising effect that has had on the Labour party, make a leadership challenge look plausible after local elections next May. Maybe sooner.

Toppling Sir Keir would be consistent with a pattern in recent British politics for short-serving prime ministers. David Cameron was the last one who managed a full parliamentary term. Since then, the average tenure has been around 20 months. This suggests there is something about governing Britain that defeats all who have tried in recent years.

That isn’t to excuse Sir Keir’s personal failings. A more charismatic leader with a more developed........

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