Last summer I was described by some as Jeremy Corbyn’s economic guru. I was not. He just borrowed some of my ideas and used them during his campaign to be leader of the Labour party even though I was not, and am not, a party member. They may or may not have helped him achieve his victory; we will never really know and that’s not the question of the moment which is, as many of his shadow ministers desert him, whether he can or should survive as leader of his party.

If I was not an idealist I would not have created the ideas that Corbyn borrowed from me. And if I was not a pragmatist I would not be writing now. My appeal at this moment is for Labour to embrace these two positions simultaneously. That is because whatever Labour’s pragmatic need might be it must be infused with a new sense of idealism. If not it is wasting its time and those fighting its internal wars will end up with the prize of perpetual irrelevance.

I do not say this lightly. If fewer people are now connecting with Labour, as seems likely, then it is wise to think there is good reason. That was very obviously true in 2015. In whatever the roles that I have, as economist, tax campaigner, chartered accountant or dad, it was clear that Labour offered “austerity light” at that election. Its only really successful day in the whole campaign was when it focused on non-doms. Thereafter the electorate could not split what Labour had to offer from the more distinct Conservative version of the same thing. No wonder the Tories won.

And no wonder so many who were seeking real difference rallied to support Corbyn and his distinctly different approach to politics. It’s not clause IV socialism. But nor is it the pro-market fundamentalism coupled to the myth of choice that had dominated the offerings of both parties for decades. Corbyn seemed like a breath of fresh air to many.

Now his leadership is in crisis. I will make myself unpopular for saying this, but I think that those demanding change are right to do so, even if I will not agree with much of their reasoning. In my opinion Corbyn has been guilty of three things. First, he has not grown into the job in the way John McDonnell has into his: after nine months he still feels like the reluctant leader who cannot do up his tie when necessary, and I hate to say it, but such messages are important. People believe that this is a slap-dash approach that means he cannot lead as a result.

Labour MP resignations: Ian Murray, Gloria De Piero, Kerry McCarthy, Heidi Alexander and Lord Falconer; Lucy Powell, Lilian Greenwood, Seema Malhotra, Vernon Coaker and Karl Turner; Chris Bryant, Stephen Kinnock, Diana Johnson, Toby Perkins, Anna Turley; Neil Coyle, Jess Phillips, Alex Cunningham, Wayne David and Lisa Nandy. Photograph: PA

Second, he has not provided a vision of what his leadership will deliver. On economics, for example, we have so far heard almost nothing that really progresses the ideas outlined last summer, and they were not sufficiently explained then as a plan people could relate to (and I say that as someone who wrote much of them, but not in the form or combination in which they were used).

And last, he has not shown the ability to accommodate those who think along other lines, which all leadership requires. I fear that suggestions that a bunker mentality was created around the leader’s office are correct. I am on record that I was not spoken to for months, despite apparently being “on side”. What of those who felt “off side” and needed persuasion? I suspect that the necessary effort to relate to them was not made. The backlash now is the result of that.

Get the person who combines that vision and that leadership now and Labour could win, even this year

For all these reasons I think Corbyn, who really is the charming, principled and genuine man so many have described when resigning their positions, is no longer the man to lead Labour. The reluctant candidate who only just made it to the ballot did not have the skills required to lead and is not going to acquire them now. And in that case a change is needed so that Labour can present a real opposition to the rightwing fundamentalists who look likely to take over the Conservative party.

What I would stress though is that there is no point in a change if Labour is not going to learn its lessons. Labour has to be an opposition. It must have a substantially different approach to the Conservatives. It must embrace the counter-cyclical investment that is so desperately needed at present in housing, business, sustainable energy and (perhaps most of all) people, who should have a right to debt-free education. In the process it would put finance and big business in its proper place, where it is treated as very significant, but not the real power in the land.

The party also has to say that outside the EU it would have the ability to create a long-term vision for a sustainable future, using (if necessary) the power of the Bank of England to create money to invest for the long term at a time when interest rates are (and are likely to remain) exceptionally low. And it must say that it welcomes migration if those who come are willing to embrace the UK as their home. Learning English, offering a skill and being willing to work where work is needed can be and should be the conditions of seeking to live in this country. Migration would be a contract, not a right, refugees and asylum status apart. Norway has done this; so should we.

Labour could be committed to jobs, education, investment and controlled migration and be true to itself, but if, and only if, it has a leader who will realise that this is what large numbers of people want from someone with the pragmatic skills of leadership needed to deliver the vision.

Get the person who combines that vision and that leadership now and Labour could win, even this year. Fail to do so and we face a bleak path towards the destruction of the state as we know it. The stakes could not be higher.