The exposure of anti-Semitism in Jeremy Corbyn’s Labour Party disproves the claim, made by many despairing Tories after the last election, that Mr Corbyn’s extremist associations can do him no harm.

I had numerous arguments with such people who told me that “young people” don’t care about his links with Islamist anti-Semites and terrorists, and had never heard of the IRA.

It was true that such attacks against Mr Corbyn got nowhere in 2017. But the key electoral point I tried to make in reply was that voters of all ages in that election did not trouble much with these questions because hardly any of them thought Mr Corbyn’s Labour would win. (They were right, just.) This meant, confusingly, that...