Brecon was a reminder of dramatic days that made reputations and signalled the death of governments

There is such a thing as a brilliant by-election defeat. There is also such a thing as a crushing loss that destroys your party

It is too early to judge whether Thursday’s by-election in Brecon and Radnorshire will be remembered as a landmark on the road to stopping Brexit, or a mere footnote in the story of how the UK left the EU.

By-elections can appear momentous but then fade into obscurity. They allow protest votes when the public are not choosing a government, and can herald false dawns. Ukip won two by-elections in 2014 when Tory MPs Douglas Carswell and Mark Reckless defected to it, but only one seat at the following year’s general election.

The contests attract the media spotlight and can be painful for candidates. The nastiest I have covered was one of my first: the 1983 Bermondsey by-election in which the Liberals beat Peter Tatchell, the gay Labour candidate.

By-elections can change events. Labour held on to Darlington in 1983; if it had lost, the party might have deposed its leader Michael Foot before the general election three months later, when Margaret Thatcher won a landslide. Conversely, the Tories’ defeat by the Liberal Democrats in the 1990 Eastbourne by-election showed the writing was on the wall for Thatcher. She was forced out by her cabinet and MPs a month later.

A pattern of by-election results can tell us something. John Major’s Conservatives lost all eight seats they defended between 1992 and 1997, when they suffered a crushing defeat by Tony Blair. There was a marked contrast between 1997 and 2001, when Labour became the first governing party for 50 years not to lose a by-election during the life of a parliament.

There is such a thing as a brilliant by-election defeat. Roy Jenkins secured lift-off for the new Social Democratic Party by running Labour close in its Warrington stronghold in 1981. He won a by-election in Glasgow Hillhead the following year.

By-elections can be momentous in different ways. David Owen’s SDP, refuseniks who had snubbed the merger with the Liberals, finished seventh behind the Monster Raving Loony Party in the 1990 Bootle by-election with just 155 votes (a feat now echoed by the remains of Ukip). We political hacks scented blood. Eventually, I got the headline I wanted: the party was over. The SDP was wound up soon afterwards.

Yours,

Andrew Grice

Political commentator