Privately, though, what senior Tories worry about the most is that Trump will go off script. One senior official confided that this was the biggest risk facing the election operation. The president can’t be trusted to stop himself weighing in with ill-judged comments, the official said.

A cabinet minister endorsed those concerns. Another senior official said measures were being taken to minimize the risk, though they didn’t say how this would be possible given Trump’s penchant for speaking his mind at all hours on Twitter, or for going off topic when he holds media briefings.

And there are good reasons for the Conservatives to be anxious.

On his previous visit to London in June, Trump gifted Corbyn an attack line by suggesting at a briefing with Johnson’s predecessor Theresa May that access to the NHS for U.S. companies would be one of the things “on the table” in trade talks after Brexit.