By elections upsets are common in the UK, but the prime minister is in peril even so — and this affects Northern Ireland

By election upsets long ago became a staple of British politics.

In the 1980s and 1990s, the Conservative governments of Margaret Thatcher and John Major suffered increasingly dramatic defeats.

In Mid Staffordshire in 1990, for example, Labour overturned a large Conservative majority from the 1987 contest in that seat. It was seen as the end of the Tories. In fact they comfortably regained the seat in the 1992 general election and indeed won the entire election, against predictions.

Seismic by elections go back much further than that. In 1962, the Conservatives won the safe seat of Orpington, only to regain it a few years later, and they have held it ever since, for more than half a century.

Now electorates are even more volatile than before and prone to lurching from one position to another. Note for example how in 2017 Jeremy Corbyn, written off as a no hoper came within a whisker of getting as many votes as Theresa May, then he disappeared off the radar again.

The Tories might well lose a by election in every single seat they now hold, however safe. Yet while too much can be read into mid-term upsets, they can be underplayed too. Boris Johnson remains in peril as prime minister.

Unionists have good reason to distrust Mr Johnson. But who else would even go as far on overhauling the NI Protocol as he seems to be doing? This worrying question needs to be borne in mind when deciding unionist tactics in getting rid of the Irish Sea border.