John Rentoul: Mr Plod comes to call at No 10

Has Tony Blair jumped the shark? It is an American phrase which means to include an over-the-top scene or plot twist that marks the moment when a television series goes downhill. It derives - apparently (I must have stopped watching the show before this) - from the Happy Days episode where Fonzie overcomes his fear of sharks by jumping over one on water skis.

As someone with a more direct interest than most in the Prime Minister's political mortality, having written a book about him, I have been waiting and watching for his moment to arrive. There were two points recently when it seemed that the water skis might truly be under him. The first came at a conference in January about the lessons the British and Americans could learn from each other's recent elections.

The showiest US speaker was a splendid theatrical production called Terry McAuliffe, a former fundraiser for the Democrats. In the middle of a fine salesman's patter about how he had turned round the party's finances, he referred to his fellow panel members, who included Lord Razzall, the Liberal Democrat campaigns chief. "Lord Razzall," he said, rolling the peerage round his Southern accent with relish. "I do like that lord title. How do you get that?"

The peer looked suitably embarrassed, but Michael Gove, the Conservative MP on the platform, said in a stage whisper: "City academies are the answer." The US visitor was puzzled, but much of the academic and political audience laughed. It felt then that the Government was inching a little closer to that territory from where recovery is impossible: ridicule.

It was only inching, and the moment passed, but it was a sign. On Thursday morning, two police officers knocked on the door of a head teacher's home in Wanstead and arrested him under the 1925 Honours (Prevention of Abuses) Act. Suddenly, it seemed that Blair's tenure at No 10 might be threatened by something more tangible than mere ridicule. John Yates, the deputy assistant commissioner of the Metropolitan Police, which is investigating the alleged sale of peerages, is pursuing his inquiries with an unexpected vigour. Yet I do not believe that this is the Prime Minister's jump-the-shark moment.

It certainly should not be. Imagine the headlines in the Daily Mail and the other anti-Blair newspapers, including, sadly, The Independent, if Des Smith, the head teacher who helped to recruit sponsors for city academy schools, had been a peace protester. Imagine the outcry if two policemen who "looked like nightclub bouncers" had knocked on the door of a 60-year-old head teacher who had, like one anti-war protester, withheld 10 per cent of his income tax because he didn't want to pay for Iraq. (The payment of taxes should, of course, be voluntary in this country.)

Yet Smith has not done anything illegal. Only something extremely foolish, which is its own punishment. He allegedly observed that if someone were to put £10m into city academies, a peerage would be a "certainty - well, almost". Unfortunately, he was talking to a Sunday Times journalist pretending to act for a rich benefactor. All this was in the news at the time, in January, including Smith's admission that he had been "naive", when he resigned the next day as an adviser to the city academies programme. Yet instead of asking him if he could help with their inquiries, the police arrested him.

Extraordinarily, an unnamed "Scotland Yard officer" speaking to The Times explained that the arrest was not aimed at Smith at all: "They are trying to go for the weaker links first, in the hope of exposing the chain of people behind the promise of honours." Is this really how British police officers think? That the patently unprovable crime of selling honours is so important that it requires the humiliation of a "weak link" in order to intimidate the big fish?

I am not a lawyer, but I can read, and it is reasonably clear to me that there is no prospect of a conviction under the 1925 law. The relevant sentence of the Act reads as follows, with the subclauses that do not apply taken out: "If any person obtains from any person, for any other person, any valuable consideration as an inducement for endeavouring to procure the grant of a title to any person, he shall be guilty of a misdemeanour." So the valuable consideration has to be obtained for the law to be broken - it is not enough merely to talk about what might happen if it were obtained.

As for the idea of successfully prosecuting one of the people higher up the "chain" - by which is presumably meant Lord Levy, the Prime Minister's fundraiser (ignoring for the moment the armchair CID's fantasy of bringing Blair himself to book) - I do not see how it could be done. It would require proving beyond reasonable doubt that a consideration (such as a loan) was obtained "as an inducement" and that Lord Levy was "endeavouring to procure" peerages.

The sale of peerages is plainly a bad idea, even if it cannot be proven and did not happen in the cases of the four nominees who were blocked by the committee set up by Blair to ensure propriety. But this police investigation is politically motivated and media driven. It is astonishing - and indeed significant - that the police should be taking it seriously.

It reminds me of a gold Citroën Saxo round the corner from where I live that says in the back window: "Taxed, tested and insured - go catch some real criminals." I used to think it an ill-judged thumbing of the nose at the police, but now I'm with the owner. What does it say about police priorities that the "crime" of putting large sums of money into state schools in deprived areas is regarded as so serious that it justifies such heavy-handed tactics?

What it says is that the police have been influenced by the media hoop-la over loans for lordships, and that they are not afraid of the Prime Minister. Nor should they be, of course. But that does not mean that senior police officers should be joining in a media-political game that has no possible prospect of securing a conviction.

Has Blair jumped the shark? Not yet. But that won't stop a lot of people trying very hard to make him look absurd. They should not be given the satisfaction, but the end of the Prime Minister's career is, as it was always going to be, a race to hold off the growing weight of our abiding culture of cynicism.