Now it's David Cameron's turn to display his one-nation credentials And this was how matters stood as of Monday morning. Since then, the Prime Minister's situation has suddenly become very much more precarious. In Manchester this week, Ed Miliband conjured up the finest piece of conference oratory since Mr Cameron's own superb 2007 speech scared Gordon Brown off calling an autumn election. Mr Miliband - just as the Prime Minister himself did so brilliantly five years ago - has brought about a change in the architecture of British political discourse. Until this week the Labour leader has been portrayed by his opponents as an uncharismatic and generally hopeless politician who, despite gaining temporary respite due to the unpopularity of the Government, was surely incapable of winning an election. Mr Cameron and his Downing Street coterie have enjoyed referring to Mr Miliband as their "secret weapon." They assumed that, like the Italian army, he would be unsteady under fire and vanish without trace when matters got serious. This talk now looks like a first-class misjudgment. Certainly Mr Miliband has points to prove. But he has established himself as a formidable Opposition leader. Mr Miliband has not simply grown in stature. An intelligent man as well as an under-rated politician, he has set a trap for the Prime Minister. It has been generally expected that Mr Cameron would use next week's party conference speech to reaffirm his status as Tory leader. This, after all, is what Conservative prime ministers are meant to do at their annual conference. All summer he has been sending out friendly signals to his heartland, hinting at fresh action on reclaiming powers from Europe, increasingly open to the idea of a referendum, keen to make it easier for employers to sack their workers, ready to allow contractors to defy local communities and build at will. Conservative high command has allowed the notion to get around that, but for the intransigence of the hated Liberal Democrats, something called "true Toryism" (a concept which, as anyone acquainted with the history or philosophy of the Conservative Party knows, carries no reality or meaning of any kind) would be allowed to flourish, and all would be well. The Prime Minister will doubtless deliver a speech with at least some of these crowd-pleasing ingredients in Birmingham. But thanks to the Labour leader's dexterity, such a lurch to the Right no longer comes without danger. It might well engender a short-lived affection for Mr Cameron in the bosoms of a certain type of backbench MP, and some activists. But this affection would not last long. More important, it would validate Mr Miliband's ambitious - and potentially deadly - assertion that Labour, and not the Conservatives, is now the "one nation" party. Unfortunately, the Prime Minister has gone out of his way, through a series of dreadful errors of judgment, to create the impression that he is the leader of a horrible public school clique. The most recent of these mistakes was the failure to sack Andrew Mitchell. Labour's Yvette Cooper was eloquent on this subject yesterday, recalling a conversation with a constituent on a council estate who told her that, "Some of the kids round here just aren't taught enough respect for the police. Mind, it doesn't help if ministers are calling them names too. There is no arguing with the shadow home secretary about this. A respectable government cannot endure the presence of a Chief Whip who has insulted the Downing Street police - and to all intents and purposes accused them of lying. These are the problems the Prime Minister needs to ponder ahead of Birmingham next week. I believe that he only has one choice. He can follow the Conservative Party into the graveyard of opposition politics, upon which so many of its members appear set, or he can return to the idealism and high hopes of the early months of Coalition. Two years ago, it offered a new kind of politics. It still does: there is a great deal to boast about already, and much work to do. Sticking up for the Liberal Democrats may enrage some Tory supporters, and does not guarantee election victory, which is now a distant prospect in any case. But it would be honourable, decent, patriotic - and provide the best available answer to Ed Miliband. Above all, it would be truest to David Cameron himself, who remains at heart a pragmatic, compassionate one-nation politician in the tradition of Stanley Baldwin and Harold Macmillan - and Benjamin Disraeli.