They say the secret to happiness is low expectations. In which case, Donald Trump has been doing his Democratic rival a big favour. In advance of next week’s debate — the first of three — the US president has all but diagnosed Joe Biden with dementia.

The former vice-president “doesn’t know he’s alive” and “can’t put two sentences together”, according to Mr Trump. It follows that all Mr Biden need do is sound vaguely coherent.

But the Democrat will have to do more than that to shift the polling numbers. In spite of the razzmatazz, US presidential debates only rarely change an election. Some of the most cited moments, such as Richard Nixon mopping his brow in the first televised debate in 1960, or George HW Bush impatiently consulting his watch in 1992, had only a limited effect. The first Biden-Trump debate may prove to be an exception.