It is hard to know which of Jeremy Corbyn’s media appearances today did the most damage. Resembling more a series of bombing raids on Labour’s remaining electoral credibility than a serious attempt to communicate with disaffected voters, Mr Corbyn dropped incendiary initiatives as if he didn’t really know or even care much about what his party policy happens to be or what damage the resulting firestorms might inflict.

Out of a blue sky, he unveiled a plan for a maximum wage, later modified to a more conventional suggestion of pay ratios, but still a startling departure from anything Labour has been associated with for nearly four decades. The leader of the Opposition also launched a bouncing bomb of a policy on migration and free movement of EU workers, much to the dismay of some of his more devoted followers; he backed striking Southern rail workers, taking out any residual hopes for Labour in the southern marginals; and he even strafed the pressing issue of Arsene Wenger, manager of a team with a substantial fan base which most likely exceeds Mr Corbyn’s own core support.

So, to put things bluntly, Mr Corbyn was all over the place. He was more all over the place, if that’s possible, than he has been ever since this unlikely, otherworldly, Chauncey Gardiner-style figure first assumed the leadership of his party. It isn’t his fault, as we should all recognise. All of his previous attempts to gain office in his party were, true to form, more protest than anything else, but this time Labour supporters placed him in an impossible and unnatural position.

Usually it is for junior shadow ministers to commit the gaffes and have their leader loftily dismiss their confused interventions as being “not policy”, closing down discussion and shooting the hare before it has any opportunity to create more distractions from the serious business of framing policy and winning elections. Not on Mr Corbyn’s watch. Instead, on the maximum wage idea, we found the shadow Work and Pensions Secretary, Debbie Abrahams, not a name that has much troubled the headline writers, putting her own leader in his place: “It isn’t a policy. He said it in the context of policy development, he said it should be something we looked at, and of course we should look at it.”

Corbyn's speech ends abruptly

Some of his more economically sentient former advisers simply described it as “idiotic”. The objections to the maximum wage are quickly summarised: it would lead to a draining of talent from the UK; it would be easily avoided by those with the resources to do so; it was tried before and found wanting. Back in the 1960s and 1970s, in Mr Corbyn’s formative time politically, Labour chancellors used to impose extremely high marginal rates of tax on higher slices of income – in effect a cap on pay. It did not noticeably reverse Britain’s economic decline then, nor do anything for inequality, and would not do so now. Well meaning, but daft is the epitaph that particular policy deserves, and, unkindly, so does Mr Corbyn’s leadership.

The Labour leader’s attempt to have the best of all worlds on immigration also hit an off-key note. Hitherto he has been all in favour of freedom of movement and hostile to migration controls as a matter of high principle. Maybe someone suggested to him that he could be a sort of leftie Trump or maybe he just got spooked by his poll ratings and came under pressure from the unions – but his U-turn on that was both unconvincing and risked alienating those Labour members who voted for him to be leader precisely because he wouldn’t trim and tack according to the polls. That’s the sort of thing Tony Blair did. They expected better of Mr Corbyn. Now he suffers the cry of “betrayal” just as his predecessors had to whenever they tried to compromise with the electorate. He cannot win, in any sense.

When will all this end? Sooner, perhaps, than some might suppose. His two leadership election victories – convincing ones – made Mr Corbyn appear all but unassailable. And yet in politics, as we have had much reason to ponder lately, the unexpected always happens. The Copeland by-election may well have been engineered by its former Labour MP, Jamie Reed, as a method of destabilising Mr Corbyn, and it could do so with appropriately nuclear impact. The home of Sellafield, it is a marginal seat, with a Labour majority of only about 2,500, and not best disposed to the CND-minded Mr Corbyn. It could easily be captured by the Conservatives, for the first time since 1935.