The Labour party is in danger more mortal today than at any point in the over 100 years of its existence. I say this as someone who led the party for 13 years and has been a member for more than 40. The leadership election has turned into something far more significant than who is the next leader. It is now about whether Labour remains a party of government.

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Governments can change a country. Protest movements simply agitate against those who govern. Labour in government changed this country. I don’t just mean the minimum wage, civil partnerships, massive investment in public services, lifting millions out of poverty, or peace in Northern Ireland. I mean we changed the nation’s zeitgeist. We forced change on the Tories. We gave a voice to those who previously had none. We led and shaped the public discourse. And, yes, governments do things people don’t like, and in time they lose power. That is the nature of democracy.

But in a thousand ways, small or large, which anyone in government can describe, being in power can make a difference to those we represent. The reality is that in the last three months the Labour party has been changed. Its membership has virtually doubled. Some will have joined in shock at the election result; many more are now joining specifically to support the Jeremy Corbyn campaign; some with heavy organisation behind them. These last two groups are not many in number, relative to the population. But, relative to the membership of a political party, they’re easily big enough to mount a partial takeover. The truth is they don’t really think it matters whether Labour wins an election or not. Some actually disdain government.

So this is directed to longstanding members and those who have joined but without an agenda. They’re still a majority and they have to exercise leadership now to save the party. It doesn’t matter whether you’re on the left, right or centre of the party, whether you used to support me or hate me. But please understand the danger we are in.

With Corbyn as leader it won’t be a defeat like 1983 or 2015 at the next election. It will mean rout, annihilation

The party is walking eyes shut, arms outstretched, over the cliff’s edge to the jagged rocks below. This is not a moment to refrain from disturbing the serenity of the walk on the basis it causes “disunity”. It is a moment for a rugby tackle if that were possible.

This is not the 1980s. This is by many dimensions worse and more life threatening. Michael Foot was never going to win a general election in the UK. But Michael was a towering figure who had been a major cabinet member in the previous Labour government. Tony Benn was never going to be PM; but he was a huge political character who again had long experience of government.

The unions in the 1980s were, by a majority, a force for stability and sense. There were constituencies so solidly Labour that nothing could shake them from their loyalty. The party that assembled after the 1983 defeat knew its direction. Maybe we didn’t know how far or how fast, but we knew, and the new leader Neil Kinnock knew, that we had to put aside the delusion that we had lost two elections because we weren’t leftwing enough and start to modernise. And our objective was to return to government.

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What we’re witnessing now is a throwback to that time, but without the stabilisers in place. The big unions, with the exception of the most successful in recent times, USDAW, are in the grip of the hard left. And the people do not have that same old-time loyalty.

If Jeremy Corbyn becomes leader it won’t be a defeat like 1983 or 2015 at the next election. It will mean rout, possibly annihilation. If he wins the leadership, the public will at first be amused, bemused and even intrigued. But as the years roll on, as Tory policies bite and the need for an effective opposition mounts – and oppositions are only effective if they stand a hope of winning – the public mood will turn to anger. They will seek to punish us. They will see themselves as victims not only of the Tory government but of our self-indulgence.

Jeremy Corbyn doesn’t offer anything new. This is literally the most laughable of all the propositions advanced by his camp. Those of us who lived through the turmoil of the 80s know every line of this script. These are policies from the past that were rejected not because they were too principled, but because a majority of the British people thought they didn’t work. And by the way, they were rejected by electorates round the world for the same reasons.

Immense damage has already been done by a policy debate defined by its irrelevance to the challenges of the modern world

Even more so today, they do not think their challenges can be met by old-fashioned state control as the way to personal or social empowerment; they do not think breaking up Nato unilaterally is sensible; and they realise that a party without a serious deficit-reduction plan is not in these times a serious contender to govern them.

I don’t doubt that his campaign has sparked interest. Why wouldn’t it? There is something fascinating about watching a party wrestle with its soul. It doesn’t mean it is a smart place to be. And, yes, some young people will be enthused. Many Young Labour members were enthused in 1997 and are enthused by modernising Labour policy today.

The tragedy is that immense damage has already been done by a policy debate that, with some honourable exceptions, is defined by its irrelevance to the challenges of the modern world. We should be discussing how technology should revolutionise public services; how young people are not just in well-paid, decent jobs but also have the chance to start businesses that benefit their communities; how Britain stays united and in Europe; what reform of welfare and social care can work in an era of radical demographic change.

Instead we’re talking about bringing back Clause IV. There is a vast array of policy questions to answer. We’re not even asking them right now. We know where this ends. We have been here before. But this sequel will be a lot scarier than the original. So write it if you want to. Go over the edge if you want. But think about those we most care about and how to help them before you do.