If Joe Biden wanted to design a gaffe to cause his presidency the most damage, it would be hard to do better than that.

This Nato summit in Washington has already been overshadowed by questions about the US president’s age and mental capacity, and every world leader has been asked about it in interviews here.

Two miles away, on Capitol Hill, Democrats have been locked in conclave all week to discuss what to do about Mr Biden and his health.

All eyes were on this evening, to see if he could bring this summit to a close without a hitch. He could not.

There was a gasp in the media centre, where hundreds of journalists were gathered to watch that press conference.

On screens around the room, Mr Biden turned to his left and prepared to give way to Mr Zelensky.

“And now I want to hand it over to the President of Ukraine, who has as much courage as he has determination,” he said.

Pausing for effect, he concluded: “Ladies and gentlemen – President Putin.”

Immediately, the atmosphere among the assembled hacks turned from one of interest, to one of horror.

The feeling in the room was exactly the same as at the presidential debate on June 27, when Mr Biden froze on stage. People looked at each other as if to say: “Can this really be happening?”

Sir Keir Starmer, Olaf Scholz and Emmanuel Macron were all asked about it in their own press conferences minutes later, and all dismissed Mr Biden’s mistake as a harmless slip of the tongue.

But the truth is that the timing of this incident could not have been more unfortunate.

With unprecedented pressure on his campaign from fellow Democrats, Mr Biden’s team have been hoping that this week’s summit was a chance to show his “big boy” credentials.

He was meant to turn up and play his favourite role: the man saving the world from Russia, and preventing a second term of what he calls Donald Trump’s “unhinged” agenda.

Instead, he made America look like an international joke, and left Western leaders scrambling to cover for him.

It is almost certain that tonight’s events will see more Democrats come forward to declare they cannot support his campaign any longer.

If Mr Biden looked shaky at the start of this week, tonight he looked finished.