The Budget will hurt. That much we know. But how much will it hurt?
Who will it hurt most? Will they tolerate the pain? And, perhaps most importantly, will it work? Will the Budget deficit disappear or will recession reappear?
The answers to these questions will shape politics for the next five years and determine the fate of the coalition. The Budget is that important.
We know that from Tuesday many of us will start paying more tax and receiving fewer public services.
Those working in the public sector will expect to have to put more of their declining wages into their pensions. Fewer people will receive benefits and tax credits.
That is what the Budget will mean for many of us.
But leave aside the detail for one moment. The key political point of the Budget is that the argument over spending cuts has yet to be had.
George Osborne had a go last year, warning of an age of austerity, but his candour was answered by plummeting poll ratings so he quickly shut up.
Showing some ankle
None of the parties engaged in more than shadow boxing over cuts in the election campaign.
The opinion polls show a mixed picture, with the public supporting cuts in principle, but rarely in practice. No one in the government really knows how people will respond to the Budget.
Will there be grumbling or riots? Letters to the media or mass demonstrations? Who knows?
Hence the desperate campaign by the government to prepare the way for the cuts.
There have been speeches from the prime minister and the deputy prime minister warning of the tough times ahead.
The chief secretary to the Treasury, Liberal Democrat Danny Alexander, has shown more than a little ankle and announced he is reversing £2bn of last-minute Labour spending pledges.
The chancellor has been all over the weekend television warning of the "road to ruin" if his cuts are not implemented, attempting to create an unstoppable momentum behind his "unavoidable" tough choices.
In other words, the Budget should be seen as the latest stage of a political argument, not just a series of rather scary numbers that will change our lives.
The government has to persuade people in the short term that what it is doing for the long term is right.
Ministers hope that the certainty of the pain to come will in itself encourage confidence in the markets. They insist the pain will be shared evenly.
Their aim is to change the mindset of a nation: that the time for efficiency savings is over and the time for government doing less has begun.
Not surprisingly Labour disagrees and spots an opportunity. Shadow chancellor Alistair Darling and others warn that cutting too deep and too fast will risk the fragile recovery and plunge us back into recession.
They know some Liberal Democrat MPs and voters share their concerns and hope to use the Budget to drive a wedge between the coalition partners.
The risk for them is that they look like irrelevent naysayers, protesting at every cut, unable to say what they would do as they contemplate introspectively who their future leader may be.
The more canny line of attack from the opposition is to ask what the government is doing to promote economic growth.
Yes, we know that corporation tax will be cut and there will be modest National Insurance holidays for companies creating jobs outside London and south-east England.
But who is going to be buying British exports when all of Europe's governments are cutting their spending? Where is all this economic growth to come from?
For all the critics' doubts, Mr Osborne knows that now is the moment to strike. The coalition will never be so popular, will never have so much political capital.
The Labour opposition will never again be so distracted and rudderless. That is why the chancellor is talking of a Budget for a parliament, so that pain now will lead to gain later.
Traditionally and literally, honeymoons used to last a month. But the Conservatives and the Liberal Democrats have well exceeded their post-nuptial time in the sun.
The shadows are drawing in and on Tuesday a new era of politics will begin.