John Rentoul: In the battle of political ideas, Brown has ceded his predecessor's advantage

Four weeks ago, David Cameron was discussing the coming political season on the sofa under the turret windows of his Westminster office. He had scored some success with an aggressive and early return to the fray from his holiday, but was now facing a big choice as the most important of his party's policy reviews was about to be published. Should he start to lay out policies, knowing that if he came up with any good ones, Gordon Brown would steal them? Or should he continue to play for time and keep the good ideas that his team were working on for later?

It has been suggested that the lather over a possible early election forced the Conservatives to come up with policies at their Blackpool conference, earlier than they intended. In all the swirl of speculation, one of Brown's motives in preparing so publicly for an autumn election was said to be to "flush out" the Tories' policies so that they could be either stolen or destroyed.

Whether or not that is true, it is not the case that the Tories were forced to bring forward their plans. Cameron had already made his strategic decision four weeks ago – before the election frenzy began in earnest, although the possibility of a snap election was one of the reasons why the Tory campaign organisation had stepped up a gear at the end of the summer holidays. As Cameron explained to his aides, if the opposition came up with policies that the Government adopted, it would make it look as if the Tories were in the driving seat.

Some of the inner circle were a little nervous. "Now is not the time for policies," one of them told me. But Cameron was thoroughly vindicated yesterday.

Four weeks ago, George Osborne, the shadow chancellor, made an important announcement. A Conservative government, he said, would stick to Labour's spending plans for the next three years. At the time, this prompted rumblings of discontent in the party. Disloyal talk of how Cameron's busy return from holiday had failed to shift the opinion polls turned to mutinous mumblings about the failure of the party to promise tax cuts. We're not going to win an election by copying New Labour, was a common complaint in the Tory backwoods. No, we're going to win it by forcing New Labour to copy us, was Cameron's confident rebuttal of his critics. And so it came to pass.

I haven't heard a Conservative argue that his party is winning the battle of ideas and sound as if he meant it since the Thatcher era. But Osborne did it yesterday. The Conservatives may be the stupid party, but they are learning. They are beginning to get it. Yesterday, Osborne said Labour had copied him and he was cheered enthusiastically by the very same MPs who, a few days before, were complaining about the party's failure to make itself different from the Government.

Of course, many of them cheered because they thought Alistair Darling was copying their new tax-cutting policies. If so, the stupid-party label will stick for a little while yet. Overall, tax and spending levels will be the same under Chancellor Osborne as they are under Chancellor Darling. Osborne's inheritance tax cuts were paid for by tax rises on rich foreigners, which sounds like a Labour policy – and which now is a Labour policy.

The atmosphere in the House of Commons yesterday was beginning to resemble that in the 1994-97 period, most notably when the Labour opposition forced the Tory Government to drop the VAT increase on domestic energy. (It was the right policy and it was wrong to drop it, but that is an argument for another time.) Tony Blair was always claiming to be winning the battle of ideas, even when he wasn't. But the impression given was one where the initiative was on the left-hand side of the Speaker's Chair, and the Government was responding defensively to it. If the Tories can keep this up, it will not be long before Norman Lamont's cutting critique, "in office but not in power", will be dusted off for reuse.

I say if, because, just as some of what Blair achieved in 1994-97 was an optical illusion furthered by Blair's confidence and chutzpah, yesterday's Tory triumph was two parts strategic judgement and one part public-school bravado on the part of Osborne and Cameron.

When Cameron made his decision to show some of his policy petticoats four weeks ago, his party machine was woefully under prepared. A campaign against NHS reorganisation blew up on the launch pad because of mistakes made in research.

With the post of head of policy only just filled by James O'Shaughnessy after the sudden departure of George Bridges, it did not seem a good time to unveil new policies that would withstand Labour rebuttal. Indeed, Osborne's tax plans were a shambles. We have had a good chortle recently at Government excuses that the taxation of non-domiciles is a bit complicated.

Ruth Kelly, the Transport Secretary, had a particularly uncomfortable time on the BBC's Question Time last week as she tried to explain that when she was a junior minister at the Treasury three years ago she had looked at this issue and failed to find a way of closing what looks to lay people like a loophole. But the truth is that it is complicated, and no one knew how much Osborne's flat-rate scheme would raise. But it would not be as much as £3.5bn, which is why Darling has had to supplement it with other tax rises elsewhere.

Osborne had done what successful oppositions can do – they have the fleetness of feet to expose problems the voters want the Government to sort out, but which governments, especially long-in-the-tooth ones have tended to shelve as too difficult. Cameron and Osborne did not have to have all the details nailed down to win the political argument. Brown and Darling could only say: Your plans do not add up; we've done them again so that they do. As Cameron predicted four weeks ago, the civil service do the sums, but the Conservatives get the credit.

j.rentoul@independent.co.uk