Robert Conquest made two great contributions to human knowledge. As a historian he revealed the truth about Stalin's purges in the 1930s. As a writer, he formulated two laws. His First Law states: "Everybody is reactionary on subjects he knows about." I don't think it is true. I would say that everyone is a gradualist reformer on subjects they understand. But Conquest's Second Law is a good guide to life, and especially politics. It says: "Every organisation appears to be headed by secret agents of its opponents."

That is how we should analyse this pre-election party conference season. The Liberal Democrats had clearly been infiltrated by a Conservative mole with a sense of humour. What would be funnier, the mole must have thought, than to have the party that wants to abolish property tax and replace it with a local income tax come out for a new tax on more expensive properties?

The Labour conference was plainly choreographed by Conservative Campaign HQ. The one thing that united the Conservatives in Manchester last week was their private desire to see Gordon Brown stay as Prime Minister up to the election. Hence the need for Labour, and Brown, to show resilience, determination and discipline.

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The strategy for inflicting maximum damage on Labour's chances required the Prime Minister to make a good, but not very good, speech, and for everyone to stop talking about the leadership. Hence Alan Johnson, the Home Secretary, read like a hostage from a script drafted by Andy Coulson, just as conference opened, saying: "I haven't got that real aching desire to lead, which really is an essential quality in a leader." It worked a treat.

As the Conservatives converged on Manchester, they must have thought, what could possibly go wrong? All we have to do is pretend to come up with some policies so that people don't say that we are completely vacuous, and David Cameron needs to make a speech that sounds serious. A good speech – and we know Cameron can do good speeches – would lift the Tories for the final straight and put the election beyond Labour's reach.

They reckoned without Conquest's Second Law. They did not realise that their conference had been planned by a Labour secret agent. Peter Mandelson, probably. Thus journalists were obsessed during the first few days with Europe. The general public is not much interested in the Lisbon Treaty, nor in the precise suspect qualities of the Tories' Polish and Latvian allies in the European Parliament. But it gave Boris Johnson the chance to grandstand, and prevented the Conservatives getting their messages across.

Then came the two big speeches of the week, George Osborne's and Cameron's; both of them brilliantly sabotaged by Mandelson. The Shadow Chancellor proposed to save money in the parliament after next by raising the state pension age more quickly. It was disclosed to selected journalists the day before the speech, and rather late in the day. This had two results. One was that much of the reporting of the plan was wrong, in that it was not until later that we realised that, while men would have to work a year longer from 2016, women would not be affected until 2020. The other was that cynical bystanders such as me assumed that the purpose in briefing the story was to push Boris down the headlines. Beyond that were two larger puzzles. One was that the Tories are trying to cut public spending at the expense, particularly, of people on lower incomes – contradicting their claim to compassionate conservatism. The other was that it won't save any money before the election after next, so it is not only damaging with low-income voters but need not be announced now.

Then there was the double agent's triumph: the leader's speech. Cameron has excelled at this before. Indeed, legend holds that he won the leadership of his party by delivering the speech of his life at the beauty-contest conference in Blackpool in 2005. Legend also has it that he stopped Gordon Brown calling a snap election in 2007 by making the speech of his life, the walk-and-talk performance when the text was engraved on the inside of his eyeballs. Given that this was an odd-numbered year, therefore, it was time for Cameron to make – well, Anthony Seldon spelt it out a month ago in The Daily Telegraph: "Cameron's speech at the party conference in early October must be the most important speech of his life."

Last week was not, therefore, the time to come up with a dud. Which is what he did. I didn't like him watching his daughter skip across the playground on her first day at school. I thought it was unwise to say: "I want every child to have the chances I had." As someone was bound to comment, that is going to cost a fortune in tailcoats. When he said: "I am not a complicated person," I thought it wasn't for him to say. I may not be the best person to judge, because I do not really like the big conference set-piece as an art form. Despite my slavish admiration for a former prime minister, I never liked even his overwrought rhetorical presentations. But nobody seems to have enthused about Cameron's speech on Thursday.

That means that Cameron is the big loser from the conference season. He was the leader with the chance to move up a gear; with a media willing him to do it and an electorate ready to listen. And what did he have to say? Big government bad; health visitors good; Labour hasn't done enough to reverse the inequality of the Thatcher years. He blew it.

And that means that it is not all over yet. The really important polls last week were those showing Labour doing better against the Conservatives than Brown does against Cameron. That means that, despite Alan Johnson's modesty and David Miliband's gawkiness, it is still worth Labour changing its leader. That is what the Tories fear. They need to be 11 or 12 points ahead in their share of the vote to win a majority in the House of Commons. I do not believe that Labour can get that close under Gordon Brown, but could do so under another leader.

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