America cannot go on like this – Joe Biden must pass the torch to a new generation

Joe Biden may or may not suffer from some undiagnosed condition, but the United States cannot take the risk that a man of his age might not function in the manner expected of its chief executive in the coming years.

The Democratic Party cannot assume that Mr Biden will beat Donald Trump in the presidential election.

And the whole world cannot contemplate with equanimity a second presidency for Mr Trump, in which he would enjoy blanket executive immunity. The stakes in any race for the White House are high, but this year they are closer to the stratosphere than for decades.

The president knew that his performance at the conclusion of the Nato summit would be a very public test of his abilities. He failed that test.

To be fair to him, he answered many detailed questions with verve and with a clear grip on the facts. His instincts are plainly sound – and certainly compared with those of his megalomaniac rival, Mr Trump. However, in front of a global audience, he introduced Volodymyr Zelensky as “President Putin”, and Kamala Harris was referred to as “Vice-President Trump”.

It was a wince-inducing disaster, and has only served to add to the already considerable concerns about the president that became so acute during his catastrophic debate with Mr Trump.

The question is no longer whether Mr Biden should be replaced as the presumptive Democratic nominee, but when and how. Time is short – the sooner a new candidate can be adopted, the better.

The only good thing, from the Democrats’ point of view, that can be said for having had that first presidential TV debate so early is that it meant the search for a replacement can be conducted relatively quickly.

Mr Biden is not quite yet the formally endorsed candidate, but the