The least important thing that happened last week was The Sun coming out for the Conservatives. We knew that already. The Sun stopped supporting Labour the moment that He Who Is Suddenly Back Among Us left Downing Street. So the timing of the announcement was the only interesting thing about it. Interesting on News International's side for its sheer self-important vindictiveness; and on the Labour side for the failure of Gordon Brown's people to ask themselves in their planning, "What can go wrong this week?"

The Sun's decision was curious for another reason. It bore no relation to the policies of either the party the newspaper used to support or the one that it now supports. In his speech on Tuesday afternoon the Prime Minister announced policies that might as well have been copied from "The Sun Says". Bring back the workhouse for teenage mothers? Cut. Paste. A new crackdown on antisocial behaviour? Cut. Paste. As it was, Brown might as well not have bothered. He might as well have announced a manifesto promise to ban Page 3. At least that would have got a cheer in the hall.

Nor has David Cameron promised anything new recently to win The Sun over. Since the "hug a hoodie" days of long ago, he has adopted the harsher rhetoric of "broken Britain", but the policies to back it up remain thin. He offers a tax subsidy for marriage, a policy lever connected to "mending Britain" by pushing on a string; and "more police officers on the street", which is almost a parody of the empty political promise.

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In Friday's Sun, he cemented the newspaper's support by using it to announce his 10 "key pledges". His other purpose was to answer the 49 per cent of people that say, in our ComRes poll today, that they "don't really know what David Cameron stands for". The Sun pledges contain nothing new, yet they do, paradoxically, give an idea of what Cameron stands for: a low-tax, benefit-cutting, nationalist populism. For Sun readers, Dave stands for the values of Sun readers. Yet when he talks to The Independent on Sunday, he couldn't care more about the environment, or civil liberties, and he's a "liberal conservative not a neo-conservative" in foreign policy.

This is opportunistic, reprehensible and the right strategy for a democratic politician. Some journalists complain that Cameron is coy about what he would do to bring the deficit down. Indeed, I have written that he could face terrible problems after the election because he has not been straight with people about the tax rises that will be needed to restore the public finances. (I am strictly impartial about this, because I have criticised Gordon Brown for refusing to spell out tax rises too, but as Cameron is more likely to be prime minister after the election, his evasions matter more.)

My argument was not that he should be honest for the sake of it, but that the hardship he will be required to inflict on the taxpaying voters is so severe that he will be a one-term wonder if he fails to secure a strong mandate from the people at the election for the tough measures to follow.

I don't know what I was thinking. Two things this week have brought me to my senses. One was The Sun's publicity stunt; the other was the Irish referendum. They both show that Cameron knows what he is doing, behaving like a penguin in response to demands that he spell out his policies. (In Madagascar the leader of the penguins tells his companions, as they escape from the zoo: "Just smile and wave, boys; smile and wave.") The last thing Cameron should do is answer questions about policy.

The last thing he should do is say that the basic rate of income tax will probably have to go up to 25p in the pound, and VAT up to 22.5 per cent. He should look serious and say, we have said that taxes "may have to rise", then change the subject to the tight control of spending and how he really would like in an ideal world to get taxes down.

Which is what he does. After all, who knows what may happen on the other side of an election? He will have to put taxes up, but he has prepared the ground as much as he needs to. He is not bound by some foolish "read my lips" promise to cut taxes; he has warned people that things will be tough. Anything more specific is asking for trouble. He is engaged in vote-winning, not economic forecasting.

Of course he should be all things to all people. They said that, as if it were a criticism, about He Who Shall Not Be Mentioned, and it never did him any harm. In fact, he started to become unpopular only after five years in government, when it was no longer possible to fool most of the people most of the time.

It is the same with Europe. Two weeks ago I wrote that the Irish referendum would be a test of Cameron's leadership. I thought he should face down the Europhobes, and tell them that, if the Tories won the election and the Lisbon Treaty were in force, they would have to live with it.

How naive of me. Tell the swivel-eyed, straight-banana brigade the truth? What was I thinking of? Tony Blair's Clause IV moment? Oh dear, I think I must have had one of those rose-tinted nostalgia moments. The fact is that Blair was so bold that he didn't even mention Clause IV in his conference speech for fear of a hostile reaction in the hall. "Say what you mean and mean what you say," he said, but he didn't even draft the form of words with which to replace the old Clause IV until months later.

This week the Conservatives will announce a lot of policy that will be trivial or recycled, to try to give the impression of a party preparing for government. Cameron's real challenge this week is simpler: to get through to the end of his party conference having made as few and as small new promises as possible.

John Rentoul's blog is at independent.co.uk/jrentoul