Fifty-five years ago last month, Robert F Kennedy was assassinated as he celebrated victory in the California presidential primary. He was the target of a lone gunman whose crime was witnessed by numerous people standing right next to the killer. It was both tragedy and outrage.

For his family it was also, naturally enough, a trauma (coming five years after the murder of his brother, US president John F Kennedy). And perhaps unsurprisingly, it is one the slain politician’s son, Robert F Kennedy Jr, has never really recovered from.

Now Jr is running for President and his campaign is one of tragedy and outrage.

Searching for an explanation for the inexplicable — the murder of his father and uncle — RFK Jr has endorsed conspiracy theories. In the case of his father’s death, common sense alone suggests such a theory is wrong. There is a desperation behind alternative accounts. It is understandable that such ideas are attractive to a bereaved son, but understanding his motive doesn’t make it right.

And now, like a lot of people who start believing in one conspiracy, RFK Jr has started to endorse other conspiracy theories. After all, if common sense and plain sight are insufficient to explain something as important as the death of his father, why should they ever be relied upon? And if sinister forces are capable of such a traumatic murder and are never brought to justice, why would you not see those forces behind, well, pretty much anything traumatic?

This is how RFK Jr has come to be viewed as the champion of the many conspiracy theories attached to vaccine use and Covid restrictions. His presidential campaign is highly unlikely to be successful, and he won’t be the Democratic nominee, but he is doing better than many thought he would, because conspiracy theorists can be found on the left as well as the right.

And everywhere they are found, they are a danger to Jews.

In my new book, Hitler, Stalin, Mum and Dad, I write about how in the 1920s, my grandfather, Alfred Wiener, saw the growth of conspiracy theories as a major part of the rise of the Nazis. This remains the case.

A new academic study conducted by researchers from King’s College London, Goldsmiths, University of London, and Arden University looked at the attitudes and political views of 2,600 Britons. They discovered that it wasn’t being left or right wing that was most associated with antisemitism. It was believing in conspiracy theories.

Once you start the think that a tight-knit group of malicious people are manipulating events, it is, it seems, a short hop to believing that those people are Jews.

This is how to understand the latest statements by Robert Kennedy. He has argued that Covid was “targeted to attack Caucasians and black people”. He went on to say that “the people who are most immune are Ashkenazi Jews and Chinese”. Defending himself, he has argued that he was just citing scientific studies, he never claimed that this “targeting” was deliberate. Just that it is a prototype for biological weapons that might be able to do the same thing.

I’m not sure that his defence really makes things better.

RFK Jr has pointed out that he supports Israel and that some of his best friends are Jews. But what he has demonstrated is how problematic it is the moment someone starts believing in hidden forces. I don’t doubt that he did not wish to be characterised as antisemitic and doesn’t think of himself that way at all. To use the favourite phrases of all conspiracy theorists, he is “just citing a study”, just “asking a question”.

Roger Waters is another example. He believes that Labour and Britain’s political debate are being manipulated by Zionists. His supporters are immensely annoyed when it is pointed out that believing that the world is being manipulated by a cabal of Zionists was the exact thrust of the publishers of the Protocols of the Elders of Zion and the people, like Henry Ford, who propagated it.

RFK’s latest remarks, however upsetting, do at least provide a steer for campaigns against antisemitism. The place to start is with resisting all theories about mysterious elites, and blobs, and cabals, and cliques that are supposed to control mankind. These may not start as antisemitic, but they can get there frighteningly quickly.

Daniel Finkelstein is associate editor of The Times.