After winning the Conservative leadership in 2005, David Cameron astutely sought to present himself as a “One Nation Tory” in the tradition of Benjamin Disraeli and Harold Macmillan. Distancing himself from the excesses of That - cherism, he vowed to resurrect a gentler, softer Toryism, one that recognised that: “There is such a thing as society; it’s just not the same thing as the state.” Having acknowledged the potency of the One Nation philosophy, his biggest error was then to relinquish it so carelessly.

It was the conduct of Mr Cameron’s government that allowed Ed Miliband to claim this mantle deftly for himself in his speech to the Labour party conference. The Prime Minister once declared, in the manner of John Rawls: “The right test for our policies is how they help the most disadvantaged in society, not the rich.” His decision to abolish the 50p income-tax rate, despite having raised taxes on the poorest, proved otherwise. The phrase “We’re all in this together”, with its false promise of a shared burden, has not been heard since. The government’s reckless reform of the National Health Service and its unbalanced deficit-reduction programme have similarly deprived it of any claim to be a one-nation administration.

The Conservatives are now in retreat in those areas – the north, Wales and Scotland – that denied them a majority at the last election. The consequence, as we stated in July, is that: “It is Labour alone that can now claim to be the true onenation party: the party of the British nation.”

In recognising as much, the Labour leader has established a powerful test for his policies and those of the Conservatives: do they serve to build one nation or two? Mr Miliband vowed to create a one-nation economy, banking system and education system. Much more policy detail will be required if he is not to disappoint expectations but the ambition is admirable. If Labour is to be a “one-nation party”, it must also drastically improve its standing in the south of the country. As Vernon Bogdanor noted in last week’s New Statesman, south of the Severn-Wash line, outside London, it holds just ten of 197 seats. If Labour is to win back some of these it must, as Mr Miliband said, be “the party of the private sector as much as the party of the public sector” and the party of “the squeezed middle” as well as of those in poverty.

The Labour leader was also right to address the threat posed by the prospect of Scottish independence. In a resonant phrase, he warned that it would leave the country worse off, “not just in pounds and pence but in the soul of our nation”. Indeed, the break-up of one of the most successful multinational states in history would leave us profoundly diminished – economically, socially and culturally.

Challenged by the ever more confident Mr Miliband, Mr Cameron should use his party’s conference in Birmingham to outline finally what he really believes. His politics, a mixture of soft Thatcherism, shire Toryism and modish liberalism, has become less, rather than more, coherent with time. In the immediate term, economic revival remains the precondition for his political revival. After allowing himself to be guided by dogma, rather than evidence, he should now be entirely pragmatic and pursue those policies most likely to produce growth.

To date, he has responded to his political woes by adopting an ever tougher line on welfare, immigration and Europe. While this may appease his party’s recalcitrant right, it will not aid the construction of a Tory majority. In his speech, Mr Miliband said he understood why voters were willing to give the Tories “the benefit of the doubt” at the last election. The coming week could be one of the Prime Minister’s last chances to convince them that they were right to do so.