A “unicorn” is a graphic expression that Brexit has added to the political lexicon, referring to a mythical entity that does not exist except in the wishful thinking of partisan politicians and their media allies. It could usefully be applied in Washington DC, to make sense of the widespread political and media theory that Donald Trump could only have won the presidency with secret help from Russia.

The report by the special investigator Robert Mueller has killed off this particular unicorn. In so doing, he has reset the political agenda. There is no impeachment process available that will supply a short cut to ending the Trump presidency. If the Democrats wish to displace him from the White House in two years, they will have to win the political argument, with a credible leader and programme. Hillary Clinton relied too much on the goodwill left over from her husband’s eight years in office, and on an agenda that took for granted a slate of liberal assumptions that many ordinary Americans did not find acceptable.