Jeremy Corbyn is an effective leader not because he is charismatic but rather because he is devoid of charisma. A statement like this requires some qualification. The point, however, is that Corbyn is a man onto whom one can project just about anything.

Thus something which comes out of Corbyn's mouth can mean the opposite when it leaves the mouth of an opponent. When Conservatives want to leave the European Union it is obviously bad, however when Corbyn pushes for the same it is a sign of unyielding principle.

Mugs with 'controls on immigration' emblazoned across them are racist, whereas much more drastic restrictions on free movement by a future Corbyn-led government are merely adhering to the democratic will of the people.

It is immigration that has this week produced the starkest example of cognitive dissonance among Jeremy Corbyn's many cheerleaders. With the rise of 'progressivism' over recent years – which has to some extent replaced an older, more class conscious leftism – a person's attitude toward immigration has become the central tenet in terms of what it means to be 'left'.

It is a lie to suggest that we 'don't talk about immigration', but to say that immigration is in some way a negative force is likely to see one cast out of the circle of the good. Hence the rancour over Ed Miliband's infamous mugs.

But as we learned over the weekend, the usual rules do not apply to Saint Jez. Speaking to Andrew Marr on Sunday (23 July), and defending his party's apparent enthusiasm for reducing immigration, Corbyn said that under a Labour government there would no longer be "wholescale importation of underpaid workers from central Europe in order to destroy conditions, particularly in the construction industries."

It is important first of all to say that there is a whiff of opportunism about some of the liberal attacks on Corbyn over this. Not least because some employers really are 'importing' cheap labour from Eastern Europe with the apparent aim of undermining British workers who have come to expect a higher standard of behaviour on the part of bosses.

Ben Judah has documented numerous cases of this phenomenon on building sites in his excellent book This is London. I encountered a similar thing travelling around the country last year for my own book.

On one occasion, a Romanian housemate of mine had been brought over to the UK by an agency on a complete pack of lies - of a flat in London and a good job at John Lewis. He had ended up packing boxes for the minimum wage in a small town in Staffordshire while he was housed with drug addicts.

Sometimes I also noticed an unusual dynamic in workplaces where a majority were migrant workers. The usual fear factor was amplified for the worker toiling away in an unfamiliar country. As a young trade union organiser in Blackpool told me, the Polish workers he was trying to persuade to join the union were "scared".

"I suppose you would be, wouldn't you?" he said rhetorically. "It's bad enough working here, in a company that's renowned for getting rid of staff left right and centre, but to do it in a foreign country it must be even more difficult."

The notion that some companies are not liable to seize on a situation where much of the workforce is temporary, unfamiliar with British law and has lower expectations, is liberal naivete on a par with the Dickens-like notion that all rich men require is a change of heart.

Yet the changing dynamic that a wider pool of labour brings with it is nothing new. There was a comparable degree of rancour among some trade unionists when women were deployed in factories to aid the First World War effort and then kept on afterwards.

In 1930 for example, the Leytonstone Branch of the National Union of Railwaymen complained that "Despite there being 1.5 million unemployed, thousands of married women are employed in industry and we call upon the government to... eliminate this class of worker."

As with migrant workers today, some of the hostility reflected wider social prejudices spewed out by right-wing newspapers. The Edinburgh Evening News had in 1919 described working women as a "class of social scroungers."

More far-sighted socialists on the other hand recognised that it was bosses – rather than women – who were to blame for any potential downward pressure on pay and conditions. The mere notion that it could be otherwise – that women ought to have been kept out – sounds faintly ridiculous to our ears almost a century later.

Unlike many of his liberal critics, Jeremy Corbyn would make it easier for trade unions to organise effectively and rid workplaces of the exploitative practices that have spread like a superweed in recent years. This too is very much a part of the immigration debate. Migrants are after all fellow human beings before they are 'hard workers' who will drive you home uncomplainingly in an Uber or pick fruit to fill the coffers of some fat-walleted CEO.

Yet there is little to cheer about in the Labour leader's apparent reversion to a protectionist socialism which sees Poles and Romanians not as fellow workers, but as competitors. That so many commentators who were previously passionate champions of immigration have gone quiet this week is a reminder of how dangerous it is to throw everything in with one, apparently saintly, individual. You start off venerating that person as 'uniquely principled' and you end up betraying all the things you once believed in.