When Richard Nixon’s presidency ended in disgrace in 1974, his mood was said to fluctuate between the suicidal and the delusional. Aides ensured that he was not left alone with bottles of pills. He drank heavily. He was full of self-pity. “Will history treat me more kindly than my contemporaries?” he asked plaintively of Henry Kissinger, his secretary of state and confidant.

Tony Blair’s premiership, which has often resembled a presidency, is ending in a manner that is also flirting with disgrace. On Thursday, the busiest news day of the year, Detective Superintendent Graham McNulty and a sergeant were ushered in the back door of No 10 to the prime minister’s den. In the next two hours Mr Blair became the first serving prime minister to be interviewed by police in a criminal investigation; this the man who had pledged to clean up politics.

What was Mr Blair’s reaction? He has not been put on suicide watch or turned to the bottle. There is no hint of self-pity, still less embarrassment. After his encounter with Scotland Yard, he breezed off to Brussels for a European Union summit, assuring Jacques Chirac that everything was fine. Then it was on to the Middle East. The inference was clear: the prime minister was glad to help the police but he had much more important things to do running the country.

In Downing Street there was relief that Mr Blair had not been interviewed under caution. His press spokesman started his first briefing for journalists after the questioning by police with a jokey “ ’Ello, ’ello, ’ello.” Downing Street should perhaps put the champagne on ice. Jonathan Powell, the prime minister’s chief of staff and a pivotal figure in the Blair premiership, is expected to be interviewed by Scotland Yard under caution. Three others have been arrested: Lord Levy, aka Lord Cashpoint, Mr Blair’s chief fundraiser; Sir Christopher Evans, the biotechnology millionaire and Labour donor; and Des Smith, the head teacher exposed by this newspaper as offering honours in return for funding the prime minister’s flagship city academies.

Among our elected leaders there is a scramble to put some distance between themselves and the investigation. Gordon Brown, who has always insisted that he has never had anything to do with party funding (he apparently never asked where the money was coming from when running election campaigns), has issued a statement attacking “unfounded allegations and smears” against him. Mr Blair, who insisted that the buck for any wrongdoing would stop with him, now argues that there was never a possibility of any wrongdoing. The proposed honours, according to his spokesman, were “expressly party peerages given for party service”, and: “In these circumstances that fact that they had supported the party financially could not conceivably be a barrier to their nomination.”

This, in so many words, is saying that in Britain in the 21st century it is possible, indeed commonplace, to buy a peerage, a place in Britain’s revising upper chamber, much as it was at the beginning of the 20th century under Lloyd George. The prime minister does not see anything wrong with that; it falls under the heading of “party service”. But, just in case, Lord Levy will be hung out to dry if he makes the link between funding and honours too explicit.

It is slippery and it stinks to high heaven. It may be that after all this no charges will be brought. If in doubt, Lord Goldsmith, the attorney-general, can be relied upon to do the right thing and conclude that a prosecution would not be in the public interest. Nobody should be tempted to draw the wrong conclusions from that. The cash for honours affair has exposed the murky morality at the heart of our political system. It must change and that change should not involve taxpayers funding political parties that cannot keep their own houses in order. Will history treat Mr Blair any more kindly than his contemporaries do? On present evidence, no.