Chris Mason: Extraordinary Caerphilly by-election humbles Westminster's big beasts

Plaid Cymru's Lindsay Whittle (left) won the by-election, giving Labour its first defeat in the area for 100 years

Amid the blizzard of nocturnal numbers any by-election generates, three stand out in Caerphilly this morning.

Labour, so long the victor of so many a south Wales political contest, humbled, pummelled, crushed - attracting just 11% of voters.

The Conservatives, so often the victor of many a UK-wide election - 2%.

Yes, you read that right - the two big beasts of Westminster politics managed just 13% of the vote between them.

Just one in seven voters backed either Labour or the Conservatives, which is extraordinary.

This contest, for weeks, was instead a race between an outfit, Reform UK, which barely existed a few years back and another, Plaid Cymru - until now the perpetual bridesmaid in contests in Caerphilly.

Between them, Plaid and Reform attracted 83% of the vote – and Plaid beat Nigel Farage's party easily.

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As so often in election contests, there is a striking human story that illustrates the wider picture.

Today's victor, Lindsay Whittle, a Plaid Cymru councillor for nearly half a century, told me he remembers standing in the crowds in Caerphilly in 1968 as a 15-year-old new recruit to the party.

The party, then, came pretty close to toppling Labour in a Westminster by election.

But here is the thing: they didn't, they were second.

Defeat at Labour's hands was something Whittle was to get very used to - he has stood for election to Westminster ten times, and stood repeatedly for election to Cardiff Bay too. He had always lost - until now.

Now in his 70s, he tastes victory for the first time.

Speaking to senior Plaid figures, they detect - and have for some time - a sense that the tide may finally be going out for Labour in Wales.

They are determined to capitalise and allow themselves to dream of governing Wales, at least as the main party, after next May's Senedd elections across the nation.

The Caerphilly contest was, for weeks, a race between Reform UK and Plaid Cymru

So, what next for Reform? A solid second place with 36% of the vote, is a solid performance for an upstart, but insurgencies remain insurgent by winning – and they were easily beaten.

It is clearly not easy for them to be the first choice "none of the above" alternative to Labour and the Conservatives when there is another party also claiming that mantle.

That is a challenge for them in Wales, as it is in Scotland with the SNP, in a way that it isn't in England.

And then there are the consequences for Labour.

Labour's heritage in these parts includes some of the most consequential names in its long history: founding father Keir Hardie, NHS founder Nye Bevan, former leaders Michael Foot and Neil Kinnock.

This morning, the prospect for Labour of heavy defeat across Wales next May - ending 27 years of continuous devolved power - looks more likely than ever.

And the questions for the Prime Minister pile up higher.

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