John Rentoul: Kindly leave the stage, Mr Plod

Blair did not pledge to be 'whiter than white' but he did say 'purer than pure'

If Shami Chakrabarti had really wanted to make an impact, she would have been wall-to-wall last week, condemning the misuse of police powers to hound an unpopular minority - in this case, Labour fundraisers. The director of Liberty and defender of human rights should have been complaining long and loud about the arrest of Lord Levy, but instead took the easy option of joining in with the conventional view that a popular minority, the NatWest Three, should not be deported to America. She is right about that, but then, so is the conventional view. It hardly needs the head of a human rights group to make such an obvious argument.

Her job is to come to the defence of the persecuted, unfashionable and despised - they provide the real test of civil liberty. It is hard to know what it is about Michael Levy that most makes him hard to defend. It is probably not because he is rich. That is not usually enough on its own. It may be Alvin Stardust, whom he promoted in the 1970s. "My Coo Ca Choo" was certainly a serious incitement to punk rock. It may be the tennis. Which is understandable. The second service rule is ridiculous: letting someone who already has an advantage have another go if they miss is more fitting in a game of hoop-la with a seven-year-old than in a sport for grown-ups.

Whatever it is, there can be little doubt that the police exceeded their powers in arresting Levy. If they wanted to interview him, they could have done so, cautioning him if necessary, which would allow them to use his answers in evidence. He had already come to the police station voluntarily for that purpose. Police sources said that Levy was less than forthcoming with documents, although Deputy Assistant Commissioner John Yates denied it when he gave evidence to MPs on Thursday. Arresting him had "simplified" the process of obtaining documents, he said. It is not at all obvious how. A former police officer I spoke to said: "A search warrant is the appropriate instrument to obtain documents. Why should a person be more likely to produce documents when arrested?"

This police officer got in touch with me in April, when Yates's team arrested Des Smith, the Essex head teacher - knocking on his door at 8.30am and taking him to the police station for 10 hours. The reason the police wanted to talk to him was because he was an adviser to the city academies programme. He told a Sunday Times reporter pretending to work for a potential academy sponsor that a peerage was a "certainty - well, almost", if he put up £10m. His arrest was unnecessary too. The police officer who contacted me drew my attention to the Code of Practice for the Statutory Power of Arrest by Police Officers: "Officers exercising the power should consider if the necessary objectives can be met by other, less intrusive means. Arrest must never be used simply because it can be used."

The behaviour of the Metropolitan Police is puzzling. I cannot believe that Yates would stage Levy's arrest the day before he was due to give evidence to a committee of MPs just to give the impression that his investigation was a serious one. Although there are Labour MPs who say it is as crude as that. "What is going on there? It is a bit of a coincidence, isn't it?" said one, who thought that Tony Wright, the committee chairman, had told Yates he would go ahead with his inquiries if the police did not "get on with it".

But there is undoubtedly an element of police politics in all this. Yates knows that prosecutions are unlikely, and therefore may want to insulate himself against the inevitable hue and cry of "Whitewash!" when that anticlimax is reached. Hence the "theatrical" arrests. No one is now going to say he pulled his punches or that the Establishment has closed ranks. In a sane world, he would have taken one look at the evidence; that is, he would have read the newspapers - and decided not to waste public money on an expensive investigation.

The significance of these arrests is that they point to the weakness of Tony Blair, once master of all he surveyed. Neither journalists nor police are afraid of him - which is how it should be, but not how it usually is. I am told that the Prime Minister occasionally lets a hint escape through his iron curtain of self-control that he feels an element of partisan bias against him on this issue. He is right. It would have been unthinkable for John Major or Margaret Thatcher to have been interviewed by the police - and not only because the information about party donors was secret then. But as he has learnt many times, being right does you no good once the political ground is tilted against you.

His problem is that he promised to be better than previous governments. What he delivered was greater openness about doing exactly what they did - tending to favour (without guaranteeing to) generous donors to his party with peerages. He did not promise to be "whiter than white", any more than David Cameron said, "Hug a hoodie" - that was a News of the World sub-editor. Or Denis Healey said he would squeeze the rich until the pips squeaked. (He said there would be "howls of anguish".) But Blair did say, "We must be purer than pure." And so, although it is absolutely not a police matter, he set himself up for a terrible backlash when he turned out to be only slightly purer than the impure Tories.

Yet shocking as the amateur theatricals of Yates of the Yard were, they were not the biggest political shock of the week. On the same day that Levy was arrested in a north London nick, Populus published an opinion poll that should have - and may still have - a more profound effect on the Labour Party. Previous polls have found that asking people how they would vote if Gordon Brown took over depresses Labour's rating. But they have compared the Brown question with the standard, open-ended "How would you vote in a general election tomorrow?" question. This time, Populus asked directly comparable questions about how people would vote if Blair were leader and if Brown were leader. With Blair, the Tory lead was seven points; with Brown, it stretched to nine. Brown as leader drives more voters to the Conservatives than he gains from the Liberal Democrats. So however bad the attrition of nine years, the Iraq war and loans for lordships have been for Blair, Labour would fare even worse if Gordon Brown took over.

Compared with a police investigation that is going nowhere, the Labour Party has a real problem on its hands.