This week's budget brings on an awesome economic and political moment. The former Labour government had already committed to a greater and faster reduction in the budget deficit than any British government in modern times. The coalition government wants to do more; to nearly eliminate a structural budget deficit of 8% of national output – some £116bn – in five years. Moreover, it wants spending cuts to take 80% of the load. No country has ever volunteered such austerity. It is as tough a package of retrenchment as the IMF imposed on Greece, a country on the brink of bankruptcy. It is twice as tough as the famously harsh measures Canada took between 1994 and 1997. It is three times tougher than Sweden's measures between 1993 and 1995. In British terms, it is immeasurably tougher than what we did after the IMF crisis in 1976 or after the ERM crisis in 1992.

If we are going to embark on such a course, there has to be a national consensus that it is right. What is proposed, if we are to believe the pre-budget speeches and leaks, is the closest to an economic scorched earth policy we will ever have lived through. If it is to work, we have to be prepared to accept not just enormous economic sacrifice, but to regard it as legitimate. There has to be complete honesty about why the measures are being taken. The reasons have to be unanswerable. The economics must be unimpeachable. The measures themselves have to be extremely skilfully implemented and seen to be fair.

This is not the case just now. Of course the structural deficit has to be eliminated. But Britain has time to make the change. Sweden took 15 years to lower some departmental spending by 20%, not the five years the government plans. We are not in the position of Greece. Britain has a diversified economy. Our cumulative national debt is not large by international standards. Uniquely, the term structure of our debt is very long – around 14 years. Most of this year's debt will be sold to British domiciled individuals and companies, so the international sovereign debt crisis has much less impact on us. The level of interest on the national debt in five years' time as a share of national output is more than manageable. These are the truths about the situation; to claim otherwise creates distrust.

I don't think either coalition partner is aware of how high the stakes are being raised, the degree to which they are unnecessarily backing themselves into a corner, and how much the ground has to be prepared before launching the country on the unprecedented path they plan. For example, each of the counter-arguments I have raised needs to be carefully argued against, not shouted down by hysterical remarks about the sovereign debt crisis or references to private lectures from the not infallible governor of the Bank of England.

Both Mr Cameron and Mr Clegg know that their popularity will fall, but if the coalition really means what it says the consequence could be much worse. The lack of necessity over what is planned could knock the Lib Dems back to where the Liberal party was in the 1950s – a party of the margins – and irredeemably rebrand the Conservatives as the nasty party. The revival of liberal conservatism and the hopes raised by this unique experiment in coalition government will collapse.

George Osborne's aggression is hard to understand. The forecasts from the Office for Budget Responsibility show that the outgoing Labour government's plans were both credible and more than tough enough to arrive at budgetary sustainability. To go beyond them with between £24bn and £50bn of extra spending cuts and tax rises, as is rumoured for Tuesday, is unconscionable and will rightly be challenged. The ground has not been laid; the economics are dubious even for deficit hawks; the support tiny; the implications dire.

It is not too late for a change of course or, at the very least, to reproduce the best of what the Canadians and Swedes did. In neither country was deficit reduction portrayed as a necessity to keep a triple A credit rating on government debt, nor as a vendetta against a "bloated" public sector, as the coalition has suggested. Rather, the measures were sold as a vital period of pain in order to create a platform for much-needed public spending growth in the future. It was important for the legitimacy of both countries' plans, as an intriguing series of essays, "Dealing With Debt", from the thinktank CentreForum sets out, that the pain was implemented by parties of the centre and centre-left who believed in public spending. The public was readier to believe the need for cuts. The story in Canada and Sweden was that aggressive belt-tightening would release more spending in future. As a result, both governments could propose short-term reductions in pensions, unemployment benefit, wider welfare benefits and public sector wages as part of the package and get grudging acceptance. In neither country was any department or area of spending ring-fenced.

There were exceptions. Both governments symbolically wanted to show their belief in spending even amid the cuts. The Swedes boosted investment in universities, science and primary and secondry education, and while cutting unemployment benefit, they provided grants for 100,000 young people to go to university. The Canadians made sure that the poorest in Canada were insulated from the cuts. They also launched a national consultation to decide on which cuts and explain the rationale. Even so, it was a hazardous exercise. As John Springford, the essays editor comments, success depended upon a buoyant world economy.

Osborne is said to have studied the Canadian experience, hence his call for a period of national consultation between Tuesday's budget and the autumn's spending review, copying what was done in Ottawa. The trouble is that the terms of the consultation preclude any genuine consultation; the assumption is that spending is bad, the state needs to be smaller, nothing is more important than a triple A credit rating, and the British way of life has to change.

It is folly. Not every penny of public spending is well spent. There has to be restraint and the deficit must be lowered. Wages, pensions and welfare transfers must take a short-term hit, as they did in Canada and Sweden. But the government should also be investing in our future. It should be raising taxes on those best able to contribute. Every department should share in the pain.

I am surprised at the Liberal Democrats. They have an obligation to their party, their tradition and the coalition to argue more fiercely for a better presented, fairer, more legitimate and more balanced approach to deficit reduction than the one that is promised. And what is proposed is no good for the Tories either. Number 10 and the Treasury believe the worst can be offset by aggressively low interest rates and more quantitative easing. They will work to a degree. But what is proposed still risks everything. Politicians pay the price with lost office. Millions of British will pay a higher price – the needless squandering of their lives.