A Labour prime minister must prove this week that he is right-wing on law and order. A Conservative leader has to show that he has a progressive social agenda. Confused by modern British politics? You should be.

That 42 days’ pretrial detention means so much to Gordon Brown shows that his government is at . . . well, sixes and sevens. A victory in the Commons on Wednesday on the extension by a fortnight of pretrial detention for terrorists began as a demonstration that the prime minister was as much the guardian of national security as Tony Blair. Then it morphed into an exercise in machismo. Now it is about survival.

The good news, according to the Politics Home website, is that Brown’s unpopularity has finally “bottomed out”. The bad news is that his approval ratings have flatlined, with 77% thinking he is doing a bad job. At prime minister’s questions, he looked more miserable and lonely than ever. The talk among Labour MPs is doom-laden.

So a Commons victory is necessary, if not sufficient, to prop up a wounded PM against party rebels. As in the bad old days of the 1970s, when James Cal-laghan’s Labour minority government hung on for dear life, the mouths of Northern Irish MPs will be stuffed with gold to secure their support. As a son of the manse, Brown will not promise dissident Muslim MPs the proverbial 72 virgins in paradise, but lavish compensation may be promised to their constituents who are banged up and later released without charge.

A solid performance by the home secretary, Jacqui Smith, at a meeting of Labour MPs may have just swung it for her boss. Even Blairites will come out in her support. “However strongly I feel about Gordon Brown I’m not going to f*** around on national security,” as one Brown critic puts it.

Yet the concessions made to pass the bill have turned it into a dog’s breakfast. We are told it is cumbersome to invoke the existing civil contingencies act to deal with a complicated terrorist plot that requires more than 30 days’ investigation of terrorist computer files by the police. Yet what can be more cumbersome than to recall parliament to vote on specific instances about extending detention without trial?

In the cartoon strip Asterix in Britain, dastardly Julius Caesar attacks only in the late afternoon and at weekends to catch the ancient Britons unawares. God help them if Al-Qaeda decides to blow up a plane during the summer parliamentary recess. Will the government really recall MPs from around the world for a vote?

And is it really wise for MPs “to test the circumstances of the threat” in each terrorist case? Terrorists are arrested without charge in the first place precisely because of a lack of evidence that can be aired in public. So on what basis are MPs supposed to vote?

While the prime minister marches on traditional Tory terrain, the Conservative leader has countered by pushing his tanks deep into Labour’s territory. His audacity is breathtaking. The goals of eradicating poverty and increasing social mobility will be wrested from Brown.

Under the ugly-sounding slogan of “the postbureaucratic age” – why not try “small is beautiful”, chaps? – the Tory leader espouses the use of the voluntary sector in tackling social problems and preaches new respect for humane state institutions such as health visitors and the post office.

Cameron promises a revolution as radical in social reform as Margaret Thatcher’s on the economy to create “the good society”, a progressive goal secured by conservative means. “Remember we are the party of Shaftes-bury, Disraeli and Macmillan, too,” adds one ancestor worshipping Tory.

The opposition leader’s own survival is assured; electoral success is likely though the polls should narrow. But he craves more intellectual credit than he gets. He would like to be written up as a statesman not a salesman. Sorry, not to have obliged.

Indeed, I have criticised the Conservatives for failing to sketch a vision of what they will do in power. Political philosophy isn’t just for pointy-heads. Ordinary voters understand it through the prism of character. People like Cameron but they want to know he stands for something; just as they once admired Brown’s consistency and fiscal rectitude before he left us with “no rainy day money”.

“We are good at communication in ways the Tories hadn’t been for a while so it is easy to say we are PR-led,” retorts a top Cameroon. “But that misses the deep thinking that is going on.” He adds “the people who haven’t missed this are on the left, like John Cruddas, Neal Lawson, Frank Field and Blair’s former speechwriter Phil Collins”. From different wings of the Labour party all are philosophical critics of Brown’s top-down politics.

The Church of England, once the scourge of Margaret Thatcher, is an unexpected ally. The former Conservative leader Iain Duncan Smith’s Centre for Social Justice’s report that poverty is being driven by a breakdown in the social fabric made a big impact with thouthful liberals. As we reveal today, a United Nations report charges that we do a terrible job of raising our poorest children. Today’s deprived child is tomorrow’s mugger: compassion must be allied to common sense. The voters worry that big state socialist solutions of throwing money at problems aren’t working. They are right.

The Tories say they will not only be tough on broken Britain but tough on the causes of breakdown. In a speech to Relate, the marriage guidance service, tomorrow, Cameron will try to bind these themes together.

A Conservative government will aim to fire-fight social breakdown through better schools and American-style welfare reforms but Cameron recognises that he will never get to the heart of the big problems if children don’t have the support of stable families. Tackling that, he says, will be the defining issue for his government.

Tax breaks for marriage and ending the “couple penalty” in the benefits system are not enough to stop family breakdown. The Tories plan a massive expansion in the health visitor service to support families when they are most vulnerable and encourage voluntary organisations such as Relate to come to the aid of marriage.

Just as privatisation wasn’t in the 1979 manifesto but everyone knew Thatcher was determined to sort out the economy and the unions, so the Tories want it to be understood what they will stand for in May 2010.

The programme intrigues many leading Labour figures. Progress, the Blairite wing of the party, has announced a series of seminars on the role of the state in the light of the political challenge presented by Cameron. Latterday Blairite ministers like James Pur-nell, John Hutton, Hazel Blears and Tessa Jowell will be to the fore. “This is our ground – Brown is surrendering it to the Tories,” groans a fellow spirit.

But Cameron should beware. It is easy to enter this territory but more difficult to retreat from than Moscow. One predecessor, John Major, was destroyed by the easy confusion between “back to basics” and the personal morals of his MPs and ministers. When does promoting the modern family become social engineering and marriage counselling state snooping?

In an economic downturn, too, expensive ideas – such as more time off for parents with babies and young children and early intervention in family failure – become luxuries. And, unless Cameron keeps to his overarching theme until the public is sick of hearing it, then these proposals will soon sound like part of the waffly “raft of measures” school of policy-making.

But the opposition has one important ally – the prime minister. Perceptive Labour figures such as chief whip Geoff Hoon and the Home Office minister Liam Byrne worry that Labour has little or nothing to say on the family. Brown actively fights shy of it.

Tony Blair knew how to defuse the issue of family support by being such a convincing dad. But then the man who sat on the breakfast TV couch last week and grinned when viewers asked him, “Why have you dumped Gordon Brown on us?” was a master of political cross-dressing.

Cameron and Brown will have to work harder to compete with the master.

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