Most people have made up their minds, and nothing you can say will change them: that’s the credo of parties such as Labour and the Democrats. Don’t challenge voters on the doorstep. Use focus groups to find out what they want, and give it to them. Follow, don’t lead. But all that’s on them, not us.

It’s true that conventional attempts at persuasion fail. A meta-analysis and original experiments by the political scientists Joshua Kalla and David Broockman found that “the best estimate of the effects of campaign contact and advertising” in US general elections “is zero”. But this says nothing about voters and everything about the useless approach of the parties trying to reach them.

Further work by the same scientists along with other people’s studies show that persuasive methods do exist. They don’t change everyone’s minds, but they can make enough difference to win elections and build a kinder, fairer, greener country. These techniques are known as “deep canvassing”.

Deep canvassing works only if you have a large army of volunteers, ideally from the community you’re trying to reach. Instead of delivering a message then scuttling away, as conventional canvassers do, their role is to connect and listen. Across conversations that might last for 10 or 20 minutes, they let people discuss their feelings. Then, without arguing or judging, they share their own experiences and ask questions (“have you ever been treated unfairly?”) that might reveal common ground.

The technique was developed by LGBT activists in Los Angeles after same-sex marriage rights were not voted into law in a........

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