9 hours ago
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Gareth LewisWales political editor
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Richard Tunnicliffe's 3,713 votes saw Labour drop to third amid a 27% swing away from the party in one of its strongholds
For Reform, who had such high expectations, there are lessons to be learned.
Despite surging across the UK, they fell short in this big test.
Their ambition to be the biggest party after next year's Welsh Parliament election has taken a knock.
There will be questions about how effective the party is at getting their supporters out to vote, as the party had been banking on a high turnout being good for them.
The turnout was 50.43% - higher than any previous Senedd election.
For Labour, this was an awful result.
If their 11% vote share is mirrored across Wales next May, under the new proportional voting system, they could be facing a wipeout.
Mark Lewis/BBC
Rhun ap Iorwerth's Plaid Cymru has breached Welsh Labour's stronghold
Before the vote, one source told me about the concept of a good defeat - something the party could work off.
This was not it.
The party's Members of the Senedd meet on Friday to start the difficult conversation of how to turn this round.
One Labour source suggested to me that the party will need a "retail offer" - something stand-out - that will grab voters' attention.
"People are desperate for material improvement to their lives," the source said.
"We can talk about improvements and we can talk about legacy issues like free prescriptions but there needs to be a big sell on something new."
The deputy first minister Huw Irranca-Davies said the party needed a "compelling story".
Matthew Horwood
Llyr Powell was predicted to be a contender for victory in the by-election
Another source said that First Minister Eluned Morgan needs to try to differentiate herself even further from her UK colleagues.
She has tried this with her concept of the Red Welsh Way, but has also spoken of a partnership in power.
Welsh Labour and UK Labour working together has its benefits, but it was never going to be easy.
Voters in Caerphilly have not bought the idea that two Labour governments working together is better for Wales.
Next May, the entire Welsh electorate could deliver the same verdict.
For now, whatever happens, Caerphilly has written itself into the pages of Welsh political history.
We are about to find out if this result is an eye-opening anomaly or a genuinely new chapter.
