PLAID CYMRU’S victory in the Caerphilly Senedd by-election is, first and foremost, a win for anti-racism, tolerance and hope against Reform UK’s toxic cocktail of fear and resentment.

It is at the same time a catastrophic defeat for Labour, which has held Caerphilly in every Westminster or Cardiff Bay election for 107 years.

Labour’s collapse is unprecedented — it has previously held Caerphilly even during nationwide routs, including 1931 when it was reduced to just 46 parliamentary seats. It is rapid — Labour had a 12-point lead over Plaid in 2021, but has now collapsed to 11 per cent of the vote, a distant third place.

And it is general. Nothing in the polls suggests Caerphilly is an isolated case. Support for Labour is collapsing everywhere. Understanding why is key to any serious left fightback — against the long managed decline of Britain’s productive industries, welfare state and working-class living standards, and against the far-right reaction to that too.

The people of Caerphilly have shown that Reform UK can be beaten, and convincingly.

Reform’s vote, at 36 per cent, was more than three times higher than Labour’s (and a staggering 18 times higher than the Tories’, whose miserable 2 per cent is in extinction territory). It is above the party’s (usually first-place) ranking in Britain-wide polls. Yet it was not neck and neck with Plaid, but a full 11 points behind.

Plaid should be congratulated on taking a clear stand against the anti-immigration hysteria of the right, including by defending Wales’s nation of sanctuary status.

That status is backed by Labour in Wales, but the party — as in Scotland — is overshadowed by its government at Westminster, which competes daily with Reform for anti-immigrant headlines.

Besides its amoral cynicism, this policy is backfiring — it doesn’t reduce the Reform vote, but drives Labour’s traditional supporters away.

Almost the last plank of Labour’s appeal to the left has been that voting for anyone else risks letting Reform in. It can’t plead that any longer in Wales, where Plaid can now say it’s Labour splitting the anti-racist vote.

And how plausibly can it plead that elsewhere? The nationalist parties of Wales and Scotland don’t factor in English politics, but polls everywhere show the momentum is with insurgents.

Some show the Greens, who have shifted sharply left under Zack Polanksi, level-pegging with Labour, and that party is growing rapidly with a larger membership now than the Conservatives.

Next month will see the founding conference of the left party announced by Jeremy Corbyn and Zarah Sultana, which despite setbacks caused by internal divisions is still holding formative meetings up and down the country that attract hundreds of participants, not the few dozen more familiar to left-wing activists.

Insurgency has been the ticket to success in British politics for over a decade. Whether we consider Corbynism, the Brexit vote or the “Get Brexit Done” election, the momentum has been against the perceived Establishment. Starmer, limping to power last year on a lower vote than Corbyn lost with, has merely placed Labour more decisively in the Establishment camp in the process, and now it like the Tories is in freefall.

So reclaiming insurgency for the left is key to defeating Reform. If Labour can do that at all, they cannot under Starmer, whose removal grows more urgent by the day.

And not simply because of his policies, but because of his politics — opposed in principle to mass mobilisation and grassroots activism, of the sort we saw from Stand Up to Racism and other anti-racist campaigners in Caerphilly who deserve significant credit for beating Reform.

That is the approach, locally led and committed to conversations and engagement, we need to take on the far right everywhere. It is unlikely one party will monopolise that electorally across Britain: it is certain that Labour cannot claim to.