It is not unusual for governing parties to be humiliated in by-elections and to subsequently recover. But the trouncing of Labour in Gorton and Denton is something special. And it is especially damaging to the career prospects of the prime minister, Sir Keir Starmer.
The electoral facts are astonishing, Despite all the talk of it being a close three way race, the Greens won with a comfortable margin of 4,402 and secured 41% of the vote, compared with Reform on 29% and Labour on 26%.
Remember that Labour secured 50.8% of the vote in the 2024 general election and had a margin over its rivals of 36.7%, on a voter turnout almost identical to yesterday's. Gorton and Denton was, on that arithmetic, one of the 10% of the seats that it took that should have been easiest to retain.
Or to put it in the jargon, it has lost one of its safest seats. And having lost Gorton, Labour could lose almost anywhere.
There are three reasons why this is deeply problematic for Starmer personally.
First, and perhaps of least importance, the campaign it fought in the constituency now looks ridiculous. It constantly made the claim that the Greens were an irrelevance in the contest and that the only way to beat Reform was to vote Labour.
Well it got one part of the analysis right: a majority in Gorton and Denton did not want a Reform MP. But they chose Zack Polanski's Greens as the vehicle to achieve that, so disillusioned are they with Starmer's government.
Second, Starmer himself - through a committee of Labour's ruling National Executive Committee - vetoed the strongest potential candidate, Andy Burnham, mayor of Greater Manchester. We'll never know whether Burnham's personal local popularity would have reversed the anti-Labour tide, and would have secured the seat. That's at Starmer's door.
Finally the victory of the Greens is devastating to Starmer's positioning of the party, under the powerful influence of his former chief of staff, Morgan McSweeney. He allowed the Greens to flourish and grow, in fact he encouraged it, by positioning Labour to the right, on everything from immigration, to the EU to the economy, because he and McSweeney saw the main challenge as from Farage and Reform.
From election night onward, this always seemed eccentric at best, since one of the conspicuous trends in that election was how well parties of the left performed. If Starmer wanted to be in power for the ten years he said was necessary to change the country, he always needed to position Labour as part of a loose informal grouping that would include the LibDems and the Greens.
Instead he set those parties up in many voters' minds as the principled alternatives to both Reform and Labour. He could have taken the oxygen away from the Greens and the LibDems. Instead he fed them.
Starmer's Labour will also have been hurt in Gorton and Denton, with its significant muslim and student communities, by his cautious approach to criticising the Israeli government's strikes on Gaza and by taking his pragmatic, friendly approach to Trump. Some Labour MPs will give him the benefit of the doubt in those respects. But most will just be frantically worried about their career prospects.
So what follows from Labour's Gorton catastrophe?
First, the pressure for a massive overhaul of Labour's political positioning and policies will become irresistible.
Second, Starmer's own credibility is now so damaged that he will struggle to recover.
If he has another roll of the dice before the local and national elections on 7 May, it's only because no credible alternative candidate to be leader is ready to launch a campaign yet.
But even members of his cabinet say that the consensus among Labour MPs is he won't lead them into the next election. And when they say that so casually, it's difficult to see how even Starmer - who prides himself on never giving up - will keep on keeping on.