Len McCluskey has now joined Donald Trump in espousing the politics of conspiracy theories

I don’t like the phrase “post-truth politics”. It implies that there was a time in the recent past when politicians debated like university professors and election campaigns were conducted with footnotes giving sources.

But something strange has happened to politics in America, and something alarmingly similar seems to be washing over the foothills of power here too.

We have long run out of superlatives to describe the arrival of Donald Trump as the least intellectually serious major-party nominee for the presidency of my lifetime. This is someone who deliberately adopted the Barack Obama birth certificate conspiracy theory to draw attention to his presidential ambitions in 2011. This was, just to remind ourselves, three years after Obama had published the certificate during his campaign for the presidency in 2008.

This week, Trump responded to the humiliation of Ted Cruz’s failure to endorse him at the Republican National Convention in the only ways he knows: by distraction and doubling down. Trump tried to draw the cameras from Cruz’s speech by making an unscheduled entrance to the convention hall in Cleveland. Then, after Cruz explained that he wasn’t going to be a “servile puppy” to someone who had been rude about his wife and father, Trump repeated the attack on his rival’s father.

“All I did was point out that on the cover of the National Enquirer there was a picture of him and crazy Lee Harvey Oswald having breakfast,” Trump said. The conspiracy theory is that an old photograph of Oswald, who killed John F Kennedy, shows him with Rafael Cruz handing out leaflets in New Orleans in 1963. Yesterday Trump went into classic conspiracy theorist mode: “Ted never denied that it was his father... I’m not saying anything… This had nothing to do with me. Except I might have pointed it out… Nobody ever denied – did anyone ever deny that it was his father? It’s a little hard to do, because it looks like him.”

I have had a bit of experience of conspiracy theorists over the years, and they are always “just asking questions”, not actually saying David Kelly was murdered or that we are ruled by lizards from the lower levels of the fourth dimension.

So far, so harmless or at least, in the case of Trump, so far away.

But recently conspiracy theories feel as if they are becoming more mainstream in British politics. Scottish National Party supporters in the 2014 referendum campaign accused MI5 of hiding the discovery of new North Sea oil fields, and of trolling JK Rowling on Twitter to try to discredit them. It was they who started the original campaign to persuade people to take pens to the polling stations, to prevent MI5 from rubbing out pencil votes – a campaign taken up by Leave campaigners in last month’s EU referendum.

Until now, these have been the obsessions of people outside positions of responsibility. Backbench MPs have taken up some of the causes of conspiracy theorists, such as Tam Dalyell, who took up a Belgrano-related theory about the murder of Hilda Murrell.

But now we have the general secretary of the country’s largest trade union to thank for legitimising some of the silliest ideas in modern politics. Here is Len McCluskey, heir to a monumental tradition of leadership of organised labour, including such giants as Frank Cousins, leader of the Transport and General Workers Union, who was appointed a member of the Cabinet by Harold Wilson.

Someone elected to the leadership of Unite, successor union to the TGWU, by hard-left cliques exploiting a 15 per cent turnout; someone who would until recently have held equivalent status to a cabinet minister, now reduced to the paranoid delusions of the small-hours frequenters of internet forums.

Three weeks ago he accused Portland, the PR company set up by Tim Allan, one of Tony Blair’s early advisers, of orchestrating the challenge to Jeremy Corbyn’s leadership of the Labour Party. McCluskey said that left-wing MPs such as Angela Eagle had been “seduced by sinister forces” and urged the BBC to investigate Portland.

Yesterday, he proved this wasn’t a one-off malfunction. He said, echoing the Scottish nationalist JK Rowling conspiracy theory, that MI5 was behind online abuse of Corbyn’s opponents in an attempt to discredit the Labour leader. Only he didn’t assert it, of course, he was just asking a question. “Do people believe for one second that the security forces are not involved in dark practices?”

And he went on: “I tell you what, anybody who thinks that that isn’t happening doesn’t live in the same world that I live in.”

He can say that again.