I have created a lavish visualisation in MS Paint to illustrate a point about moderating one’s political position to try and appeal more to public opinion.
Imagine that, for a particular contested issue, “the status quo” is how things currently stand (for example, the current state of the asylum system or the welfare system). Your political aim is to move away from that status quo towards a better state (for example, towards more humane treatment of refugees or people with disabilities). But “public opinion” (whatever exactly that is) is pulling in the other direction (towards, e.g. harsher controls on refugees, more punitive treatment of welfare claimants) and your political opponents are positioned on the same side.
One view is that in this situation, you need to move your own policy position closer towards public opinion so that, by winning the support of the public, you are in a position to implement policies that, while they are less close to your political aims than would be ideal, are far closer to them than those of your opponents, which are what will be implemented if you squander power by sticking to your principles.
Now, there might be something to be said for proposing policies that are a more moderate step away from the status quo in the direction of the state of affairs you ultimately aim to reach (point B on the diagram) in order to appeal to mainstream opinion. But if you position yourself on the other side of the status quo (point A on the diagram: the same side as “public opinion”, but moving less far in the opposite direction from your political aim than your opponents) you have conceded defeat from the outset by adopting a position that goes in the opposite direction from what you are aiming at. Going more slowly than your opponents in the wrong direction is a counterproductive way of trying to get where you want to go. What is more, in the absence of any movement in the direction of your political aims, public opinion is only going to drift in the opposite direction.
For this reason (alongside others with a less consequentialist flavour), the view represented by Jeremy Corbyn is fundamentally correct: for there to be any prospect of progress towards your political aims (and this applies not only to electoral politics), you do need to stand for a political position that does not cede ground to your opponents but is, however small, a step in the right direction; and however difficult it may be, if “public opinion” is opposed to your aims, you are faced with the task of attempting to shift that opinion.
Where there are questions are about the specific implementation of this general task by Corbyn and others: how far exactly should point B be positioned from the status quo? What are the best ways to appeal to and persuade people who are (more or less) opposed to your aims? Can this be done at all through the prism of electoral politics? What are the right ideas and people for this project? But one thing is clear: if your ambition is to do anything more than apply a brake to a constant rightwards drift, you cannot position yourself (however “moderately”) on the side of harsher policies on immigration, welfare, equality, civil liberties, bombing Middle Eastern countries, etc.