Whether defending him against troubling questions about his associations with representatives of sundry terrorist organisations or explaining why he could so often be found voting against past Labour governments, Corbynistas frequently rely on the line that their man has always had right on his side.

This revisionist take (no, Corbyn did not meet the IRA while playing a crucial role in delivering peace in Northern Ireland. He was nowhere near the peace process) might baffle non-believers, but for those who have emotionally invested themselves in the Leader of the Opposition, it’s more than enough to allow them to maintain the fantasy that he is a paragon of moral purity.

I wonder how many of those supporters will – after yesterday’s extraordinary march in London in support of a second EU referendum – have the decency to, at least, die a little inside the next time they parrot this claptrap about the man who would be prime minister.

The leader of the Labour Party has subjected supporters to a rollercoaster ride. Picture: Stefan Rousseau/PA

Corbyn’s boycott of yesterday’s protest is nothing less than a betrayal of the very people – the vulnerable, the poorly paid, the powerless – who will be hit hardest by the United Kingdom’s departure from the European Union. What’s more, it’s a betrayal of his own party which – in theory, at least – is in favour of another referendum on EU membership as a way of breaking the deadlock on the question.

Corbyn’s closest political ally, shadow chancellor John McDonnell, explained that the reason he could not bring himself to attend yesterday’s event was that by doing so he might “alienate” some of the people he wanted to “bring on board”. Maybe this was good enough for the faithful, but it really shouldn’t have been.

On every political issue, there will always be those who take opposing views. If we apply McDonnell logic, those with whom a politician disagrees should never be challenged. Can you imagine a Corbynista arguing against protesting against, let’s say, restrictions on trade union rights on the grounds that to do so might alienate those who are in favour? No, of course you can’t.

Across the broader left, arguments against participation in yesterday’s march were no less infantile.

On Friday, Jonathon Shafi, co-counder of the Radical Independence Campaign, tweeted a list of participants in the march. These included Tories such as Michael Heseltine, Justine Greening and Dominic Grieve and Liberal Democrats Vince Cable and Jo Swinson. “How anyone on the left can countenance supporting this,” he wrote, “I have no idea.”

One could easily draw up a list of those opposed to the objective of yesterday’s protest – Prime Minister Theresa May, for example, and former Ukip leader Nigel Farage – and ask how anyone on the left could countenance not participating in the march.

The hard left – the Jeremy Corbyn left – has long been every bit as Eurosceptic  as the right. They’re just less honest about it.

Corbyn’s pitifully weak role in the Remain campaign in 2016 confirmed what anyone who had paid the slightest attention to his career already knew. He believed the EU was a malign force. Corbyn’s absence from yesterday’s march was not because there were Tories there and nor because he feared alienating Leavers, he was not there because he is the left cheek of the anti-EU arse. It is as simple as that.

And so, as has become a modern tradition, it fell to Corbyn’s deputy – Tom Watson – to stand up for the values the Labour Party is supposed to hold dear.

Treating his leader with the contempt he deserves, Watson said he was backing a second referendum as the only way to solve the Brexit crisis.

He would be participating in – and speaking at – the Put It To The People march because the current parliamentary impasse was not working for either Leavers or Remainers.

“I’ve come to the reluctant view that the only way to resolve this, and have legitimacy in the eyes of the public, is for the people themselves to sign it off,” said Watson. The only chance of bringing the country back together was, he added, to give everyone a “final say” and then live with the result.

To the screeching fury chimps who pollute the internet with abuse of anyone who dares criticise Corbyn, this will have been further confirmation that Watson is impure. I mean, he went on a march with Heseltine – he must be a wrong ’un, mustn’t he?

The SNP has often deployed the “standing with Tories” missile to attack Labour politicians who oppose Scottish independence but, yesterday, senior nationalists – including party leader Nicola Sturgeon – were happy to do precisely that.

Sturgeon and her colleagues could find it within themselves to unite with political opponents in pursuit of a common goal, but for adherents of the Church of Corbyn such pragmatism is heresy.

MPs may have voted to reject a No Deal Brexit – and even Corbyn says such a scenario would be a mistake – but unless they can agree an alternative that’s acceptable to the EU, it remains a distinct possibility.

Corbyn supporters might indulge the fantasy he can win an election that hasn’t actually been called and then strike a deal which works for the UK, but the reality is that they are fellow travellers – whether they like it or not – of such Tory Brexiteers as Jacob Rees-Mogg and Dominic Raab. They are standing by, doing nothing, while Brexit looms, threatening jobs and diminishing the UK in the eyes of the rest of the world. That’s not a display of moral courage but of craven capitulation.

Corbyn’s leadership – if one can characterise it as such – of the Labour Party has been quite the rollercoaster ride. Elected as the champion of a new, gentler, kinder, more compassionate politics, he was long ago exposed as a drab, unpleasant little man whose ideas were forged – and remain – in the foreign country of the past. Supporters may tell themselves his boycott of yesterday’s march was a display of integrity but, in fact, it was just the latest in a long line of moments when Jeremy Corbyn was on the wrong side of history.

Euan McColm: History unmasks Brexit fellow traveller Corbyn