This weekend, politicians at Westminster begin the painful task of recovering from the referendum, seeking to restore some sense of party unity after months of name-calling and insult. To judge from US experience, they may find it easier once they identify a common enemy

This weekend, after the EU referendum tally is due to be announced, while many Ministers busy themselves with the policy consequences of the vote, a different conundrum is also confronting the politicians: how to reunite the Conservative Party after a divisive battle over Brexit?

For Americans, this sort of thing happens every four years or so. Since the McGovern-Fraser Commission on Party Structure and Delegate Selection rewrote the rules governing the selection of candidates in the early 1970s, both Democrats and Republicans select their presidential nominees through a complicated process of primaries and caucuses, in which the voice of the voters is paramount.

Candidates of all stripes can throw their hats into the contest, hoping to vault themselves to the front of the pack, mindful of the fact that eventual Presidents Jimmy Carter, Bill Clinton and Barack Obama were scarcely known outside their home states at the time they entered the fray.