Have you seen those funny maps that stretch or compress according to the importance of the terrain they cover, not its geographic size? Try applying such elastic cartography to our understanding of the Budget yesterday. Redrawn thus, Alistair Darling's piddling measures on Jobcentre Plus would be an almost sub-microsopic islet; you'd need a magnifying glass to see his frivolous car-scrappage bonus; and his offshore wind-farm subsidy would be a speck in the ocean.

But his borrowing forecasts would straddle the map like a huge, dark continent, all else crowded to the margins. And towering above the small hills and minor ranges of “efficiency savings” and pie-in-the-sky drives to cut waste would be the Everest of public spending.

Forget the eye-catching initiatives. Forget the wind farms, the tiny tax tweaks, the trivial holidays in stamp duty. Forget, too, Mr Darling's happy thoughts for what growth might be in 2011, or 2015 or 2093. Nobody knows these things and nobody believes him. The one central truth is the mess into which public finances have descended; David Cameron did not exaggerate. And the central challenge that stared the Tory benches in the face yesterday is how to bring down state spending before it destroys national solvency.

For once, backbench opposition outrage was not synthetic. For once, massed Tory shouts were genuine. Now their front bench must respond in a way that does not shy away from what most people who might vote Conservative really feel: that government - and any government the Tories lead - is going to have to harden its heart and tighten its purse strings; that it is not enough for a party to be loved; and that it's time for Mr Cameron and his team to move on from Operation Soft Soap to Operation Flint.

Budget 2009: A step-by-step guide

Alistair Darling has revealed the steps he believes will drag Britain out of the worst economic slump since 1946

• Jobless hits 2.1m as public debt soars to £90bn

• Budget reaction: chorus of disappointment

• Alistair Darling's Budget tax hike for the rich sets election battle line

BACKGROUND

• Home sales fears despite stamp duty extension

• Power boost to wind and oil

• Wealthy pension investors hit

• David Cameron bites back at 'fictional' Budget

• Budget 2009: speech in full

There have been encouraging signs, repeated yesterday, that George Osborne and Mr Cameron are ready. If, as Mr Cameron said in a fine, fighting Budget response, Britain's public finances are in deep trouble, it must follow that tough measures are needed to rescue them. Any opposition with a moral claim to form the next government must now confront the Government on how Britain is to finance its spending, and how ambitions for our public services can be brought back into line with our capability to finance them.

What and how can we cut? What and how can we restructure? How can we protect and build on Margaret Thatcher's and John Major's legacy of resistance to vindictive, destructive taxation? It is said of Mr Brown that his aim is to establish a clear dividing line with the Tories, and I think that is true. From this it is argued that the Tories should not be so foolish as to play his game. I reject that. There are times when, if a gauntlet is thrown down, one should pick it up and draw swords.

If the Tories believe that government cannot spend and borrow its way out of the hole that yesterday's Budget described, if they believe that, given the reins of power next year, harsh accommodations would have to be made with budgetary reality, if they believe that shin-kicking populist tax increases on wealth creators will not help the poor but hamper recovery, and if they have the moral and intellectual confidence that this is an argument that can be persuasively made, and the voters can understand, then they must find the courage to make it. To break cover with such talk only after winning an election would not only be dishonest, but rob them of the mandate that they will need.

This Budget has rendered in stark relief the dividing lines that - beneath the PR patina - we know exist between natural Conservatives and natural supporters of the Left. The Tories should not feel sheepish about that divide, but proud of the side they stand on. When will a Conservative opposition find a better basis than yesterday's Budget for taking this stand?

It is possible in politics to box too clever. Because new Labour has boxed very cleverly for the past 12 years, and knocked much stuffing and self-belief out of the Conservative Party, some Tory strategists have become hypnotised into a sort of stupefied acceptance of the new Labour analysis of modern politics. If (they figure) Mr Brown wants a scrap about public spending, or taxing the better-off, then he and his advisers must know what they're talking about; so we Tories would be fools to fight him on that ground.

But what these Conservatives have not considered is that this analysis might simply be wrong: out of step with cruelly changed times. How cruelly times have changed was made plain yesterday. Voters can see it. I believe the Brown analysis is wrong. As battle lines are drawn up over this Budget, and if the Prime Minister wants to invite his Tory adversaries to choose public spending and the taxation of wealth as the battlefield, I would recommend the Opposition to accept his invitation.

“Yes,” should be their response, “you're right: we really are Tories, underneath. No indeed, just as you say, we haven't changed our spots. We still believe the State should limit what it takes from the individual taxpayer. We still believe that taxing success is destructive; we may or may not accept your 50p tax rate temporarily, but we don't like it, we don't call it social justice, and we don't think it will prove a solid source of substantial revenue.

“We're relaxed about admitting this. You can't believe your luck, you say? You want us to step outside and repeat it, to the crowds if necessary? Fine. Here's the door. There's the crowd. Come with us and let's argue this out in public. We're not ashamed of our philosophy and we think the voters are ready to hear it.”

Tory strategists are right that this is precisely the ground on which Gordon Brown wants to fight. Where they are wrong is in concluding that that is a reason for ducking. It is a fight the Tories will win.

Times Recommends

• Fragility always gets my sympathy vote
• Double it and you get the real jobless total
• Has al-Megrahi paid his council tax?