Unlike Chris Grayling, the putative Conservative home secretary who invoked a subtle television series to score a clunking political point only then to be exposed as not having watched much of it, I have seen every episode of all five seasons of The Wire, the compelling HBO series about drugs, gangs and politics in Baltimore. To him and all his colleagues in the shadow cabinet, I particularly recommend the final two seasons as a taster of the horrors that await them should they find themselves in office.

Mayor Tommy Carcetti is elected on a central pledge to crack down on crime. He moves into City Hall only to discover that his predecessor has bequeathed him a hidden, massive hole in the schools budget. To deal with this crisis, Mayor Carcetti is forced to betray his promises on law and order with the inevitable consequences for his credibility, the crime rate and police morale. In an attempt to divert resources back to their department, well-intentioned but desperate detectives resort to mutilating dead bodies to create a panic that a serial killer is murdering homeless men.

We will see if a Conservative government comes to such a macabre end. What we already know is that an enormous budgetary challenge will face David Cameron. Except it will be worse. The Conservatives will not even have the excuse that the hole in the finances was concealed by their predecessors, because everyone knows that the government is running a prodigious deficit which will have to be brought down whoever wins power. The tax and spending – or, rather, tax and cuts – question will be central to the election campaign which will unofficially begin as politicians return from the sun loungers.

Before the August break, the Tories got the better of this battle. They succeeded in making the dividing line one between their purported candour about the national finances and Gordon Brown's stubborn refusal to acknowledge that cuts are coming. This caused despair not just to Alistair Darling, but to several senior members of the cabinet. One cabinet minister, who is already giving a haircut to parts of his budget, observed to me the other day: "I'm not going to lie about it." He would be instantly found out. The prime minister's colleagues believe they have now persuaded him to move to a more defensible position. Labour will acknowledge there will be cuts to turn the argument with the Tories into a debate about how, where and when those cuts should fall.

So long as Gordon Brown persisted with his transparently untruthful claim, there was less attention paid to the essential dishonesty of the Tory position. At the moment, David Cameron and his team speak with two tongues. One Tory tongue declares that they will make deep cuts and asks us to salute the bravery of the Conservative leadership for saying so. "The public finances are awash with red ink," he cries. "George Osborne and I have been straight about the need to sort out the public finances... spending plans need to be reduced." Yet his prospective ministers are simultaneously talking from a contradictory script.

The other Tory tongue sprays us with implied promises that there will be more money for services under a Conservative government. From law and order to schools and hospitals, from defence to transport, Tory spokesmen and women routinely suggest that life will be radically improved once they are the owners of ministerial limousines. Mr Grayling introduced The Wire into our debate to promote the Tory trope of "Broken Britain". He was factually wrong to suggest that "many parts" of British cities resemble the murderous streets of Baltimore.

A second error, I think, is tonal. Tories misread the voters when they describe Britain in such dystopian terms. There is a lot wrong with the country and people know it; their government has disappointed them in various ways and people are duly disillusioned. Labour's abysmal ratings in the opinion polls tell us that. My sense, though, is that most people do not believe that they are living in a broken country.

Whenever the Conservatives seek to exploit anxiety about crime and disorder, they suggest that they will be spending a lot more money on the police and prisons. Mr Grayling did not explicitly promise us that there will be x thousand more police officers and y thousand extra prison places when he is home secretary. Like all members of the Conservative frontbench, he has been forced to take the vow of omerta when it comes to numbers. He is not allowed to say directly that he will spend more, but he certainly implies it when he claims that Britain is broken and the Tories will fix it.

Many Conservatives have been playing this game over the summer. David Willetts, who speaks for his party on higher education, is one of the most thoughtful members of their team and not as natural an attack dog as the shadow home secretary. But the cerebral Mr Willetts has been snapping around the government's ankles for failing to provide enough university places to satisfy demand. He's not wrong; it is a crushing disappointment to many young people who have worked hard to secure excellent A-levels – even straight As – and yet can't find a place at university. Mr Willlets seeks to position himself as their champion and every time he does so he implies that a Conservative government will provide the places which Labour has not.

Whenever anything goes wrong in Afghanistan, we can be sure to find Liam Fox parachuting in front of a camera. The shadow defence secretary machine-guns his scorn for a government that has compromised the mission in Afghanistan. "We are short of infantry and special forces," barks the quick Fox. Those troops we have "struggle with inadequate numbers of helicopters and armoured vehicles". I don't argue. New Labour has too often wanted to send men and women to fight wars without providing adequate resources for the job.

"Neither Blair nor Brown had the courage to take the tough decisions to match our resources to our commitments," says Mr Fox and invites us to believe that he would not be such a coward. So what would he do? Pull out of Afghanistan? He says not. Find money for more helicopters, armoured vehicles and troops from elsewhere in the defence budget by scrapping Trident? That would be "madness" he says. When the prospective Conservative defence secretary is asked to explain what he would do and where he would find the money, he mentions a strategic defence review – a foxhole for Mr Fox.

Michael Gove and Andrew Lansley want us to believe that the nation's schools and hospitals will be transformed for the better in their custody. The latest press release from Theresa Villiers, the Tory voice on transport, suggests that a Conservative government will finance high-speed rail lines to the north that Labour won't pay for. Good news for those, like me, who are enthusiasts for high-speed rail. But where's the money, honey?

In speech after speech, David Cameron says the axe is sharpened. In speech after speech, George Osborne practises his swing. The Tory leadership does not argue with the figure of a 10% reduction in real terms, which would be a spending squeeze without precedent in postwar history. Have they told Chris Grayling, David Willetts, Liam Fox, Andrew Lansley, Michael Gove, Theresa Villiers and every other Tory frontbencher who can't open his or her mouth without implying a spending increase?

Messrs Cameron and Osborne are clearly conscious of this contradiction because in their most recent speeches they've been trying to square it away. The Tory leader talks about a "new approach to public spending" which will "get more for less". How this trick works is known only to members of the Magic Circle. The shadow chancellor, in a cheeky contribution during August, went so far as to claim that "progressive reform with the Conservatives" would produce a big reduction in spending while "cuts on the frontline can be avoided". All gain and no pain! The axe that never hurts! That earns an A for audacity, but gets an F for credibility.

I grant them that money isn't everything when it comes to performance. It can be possible to improve standards without additional resources. There is nearly always waste to be identified and methods of deploying existing money more productively. It is even sometimes possible, when reform is terrifically smart, to squeeze more from less. Yet significant reform often also involves extra cost, at least initially. Michael Gove's ambition is to create many more academy-style schools, which are more expensive than average schools. Reductions to the civil service sound painless for everyone else, but they will not be cost-free; there will be a big redundancy bill to pay.

No British government of any complexion has ever managed to make a real term cut to public spending of 10% and at the same time miraculously generated more police officers, extra high-speed rail links, a larger and better equipped army, improved healthcare and better schools.

The Tories are either conning themselves or they are trying to dupe the voters – it is probably a blend of both – when they suggest that you can have both deep cuts and better public services. As they say in The Wire: "A lie ain't a side of a story. It's just a lie."