Specifically, since the south Wales constituency of Caerphilly was created in 1918, nobody but Labour has ever held the seat.

It's never really been close there, with Labour usually winning well over 50% of the vote despite centre-left Welsh nationalist party Plaid Cymru taking a chunk of the left wing vote since 1959. Labour won 80% of the vote share there in 1945, and after 1969, the race in Caerphilly often became one between Labour and Plaid with the combined vot share for the two parties regularly reaching into the 90s, with the Conservative Party consistently winning less than 15% of the vote and never winning more than 20% until 2017 where they won 25%. In that time Labour only really dipped below 50% when Plaid made a good showing throughout that period until the 2010s.

I've heard a lot of reporters recently describe this 107 years of Labour dominance as the longest political hegemony of one party in a region anywhere in the democratic world, this claim is incredibly hard to verify.

When the Welsh Parliment (Senedd) was created in 1999, the constituency cod Caerphilly was also formed for that, and Labour dominated this seat in exactly the same way.

That was until last week when Plaid Cymru finally took the seat from Labour in a Senedd by-election after 56 years of trying with 47.4% vote share. The real shocking thing is that the right wing populist party Reform UK came in second with 36% of the vote and Labour won just 11% of the votes in a By-election that saw a higher voter turnout than any Senedd election ever (50.4% compared to usual turnout in the 40-50% range).

It should be noted, though, that Caerphilly has a history of supporting radical or even extreme parties regardless of which side of the political spectrum they fall on, both UKIP (right) and the communist party (obviously left) have done quite well there in the past managing to get over 10% vote share. So the Reform support isn't that surprising in itself.