Our polls were good at identifying the correct challenger in key seats. But voters needed more than a common enemy

In the end, fears of a Corbyn government and disdain for Jo Swinson’s election campaign combined to defeat tactical voting. Over the past four weeks, the Observer has reported on constituency polls designed to assist voters wanting to oppose the Conservatives and stop Brexit. Last Sunday, we recommended candidates in 50 seats. Our results were not great. Non-Conservatives won only 13 of these seats; of these, nine were SNP gains in Scotland. In England, our preferred candidates triumphed in only four seats: Putney and Portsmouth South (Labour) and Richmond Park and St Albans (Liberal Democrat).

Why so few successes? It was not the fault of Deltapoll’s data. Almost everywhere, they identified the correct challenger. Most dramatically, they showed rightly that the Liberal Democrats were snapping at Dominic Raab’s heels, despite the foreign secretary’s apparently impregnable 23,000 majority.

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In a number of our 50 seats, the candidate we showed running third saw their vote squeezed. Examples were Labour in Wimbledon and the Cities of London and Westminster, and the Lib Dems in Putney and Hendon. In some places, divisions in the tactical voting community helped the Conservatives.

Three high-profile defectors to the Lib Dems suffered from local voters receiving conflicting, and sometimes changing, advice: Luciana Berger (in Finchley and Golders Green), Chuka Umunna (Cities of London and Westminster) and Sam Gyimah (Kensington).

Facebook Twitter Pinterest Sam Gyimah came third in Kensington, after initially looking the main danger to the Conservatives. Photograph: Steve Taylor/Echoes Wire/Barcroft Media

In Kensington, judgement was particularly tricky. We backed Gyimah when our survey showed him well-placed to win. A later survey by the same pollster showed Emma Dent Coad, who gained the seat for Labour in 2017, overtaking Gyimah. This trend proved to be more powerful than the contested tactical advice: Gyimah ended up third, and Dent Coad lost the seat narrowly.

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Deltapoll’s data showed what tactical voting was up against. Lib Dem supporters were reluctant to help Corbyn become prime minister. Likewise, Tory Remainers feared voting for a badly led Lib Dem party that might support Corbyn. These Tories preferred a Brexit Britain governed by Johnson to a stop-Brexit government led by Corbyn.

The big lesson is that tactical voting needs not just a common enemy, but a broadly common vision, shared by the Labour and Lib Dem leaders. This was the case with Tony Blair and Paddy Ashdown in 1997; it was not with Corbyn and Swinson last week.

In all this, one success deserves to be recognised. Seven of the 11 national eve-of election surveys, including Opinium, the Observer’s Britain-wide pollsters, predicted the support for every party to within two percentage points. One cannot reasonably ask for a better performance.

Peter Kellner is a polling expert and former president of YouGov