Towards the end of his speech about race on Tuesday, Barack Obama made an observation that was raw enough to knock any attentive American listener out of his chair.

Mr Obama was talking about one of his campaign volunteers, a white woman in her 20s, who as a girl had proclaimed that her favourite food was mustard sandwiches, in the hope of making her single mother feel less bad about being poor. This girl had kept her faith in other people, Mr Obama said, even though “perhaps somebody told her along the way that the source of her mother’s problems were blacks who were on welfare and too lazy to work”.