Sleaze. The word has never been properly defined. Strictly speaking it refers to a kind of shoddy cloth produced by Silesian cotton mills in the 19th century. In politics it means kind of moral turpitude. A willingness to exploit public office for private gain which stops short of actual criminality.

Sleaze became indelibly linked, or “conflated” to use the word of the moment, with the Conservative Party in the 1990s.

It was a prime factor in the downfall of the Tory prime minister John Major. His MPs were discovered not only to be taking cash for questions but almost routinely offering to act as hired guns for commercial interests. That led to the Nolan inquiry and the Committee on Standards in Public Life.

Now history is repeating itself as farce. Boris Johnson has single-handedly revived one of the greatest Tory negatives since Theresa May called them the “nasty party” in 2002. By attempting to defend the indefensible commercial lobbying by one of his MPs – and then un-defending it – the Prime Minister has made his administration appear both corrupt and incompetent.

The affair has been compared to Dominic Cummings’s trip to Durham during Covid, but it is much worse than that episode. Cummings was at least trying to protect his family. Owen Paterson was just protecting his bank balance. Promoting the interests of private firms in Parliament is not just against the rules. It is one of the worst offences an MP can commit. Mr Paterson had been caught bang to rights lobbying for firms which had been paying him £100,000 a year.

Many voters were surprised to learn that Members of Parliament are still allowed to earn large sums from private firms. MPs are paid £82,000, plus expenses, from the taxpayer. When that increases next year it will be three times the average UK salary. Not surprisingly, there are renewed accusations of parliamentary moonlighting.

After all, private firms aren’t daft. They pay MPs to promote their interests. And even if there is no formal lobbying of ministers going on, there is still a potential conflict of interest. It is extremely hard to justify any lucrative involvement with commercial firms.

We may indeed have seen the last of this gravy train.

For the handling of this scandal can only be described as a political and reputational disaster – and Boris Johnson is up to his neck in it.

His ex-consigliere, Dominic Cummings, likes to refer to the PM as “the shopping trolley”. Last week, it finally went off the rails. Boris Johnson’s worst character flaws were on public display.

His impetuous and unreflective decision-making; his lack of respect for the rules; his irresponsible cronyism.

After ordering his MPs to back a three-line whip effectively scrapping the standards committee that polices the lobbying activities of MPs, he had an overnight change of heart. The hapless Leader of the House, Jacob Rees-Mogg, was sent to the depatch box to conduct a screeching U-turn.

“The case of Owen Paterson and the reform of the standards committee should not have been conflated,” said Education Secretary Nadhim Zahawi. “Conflated” seems to be a euphemism for a copper-bottomed balls-up.

Mr Johnson’s change of heart may not have been unconnected with the furious criticism of the Paterson affair in normally Tory-supporting papers like the Daily Mail. Even The Daily Telegraph, which had expressed some sympathy for Mr Paterson, whose wife committed suicide in June, was mightily dischuffed.

Tory papers have anyway been fast losing confidence in Johnson. His high public spending, his manifesto-breaching tax rises and his evangelical environmentalism are not to the taste of mainstream conservatives. Moreover, it was The Daily Telegraph that exposed the “duck house” expenses scandal 10 years ago that led to the current rules being tightened up.

MPs were then found to be spending thousands of taxpayer pounds on moats, second mortgages and lavish furnishings. Several went to prison. There has been a zero-tolerance culture in Parliament ever since.

This for the very obvious reason that voter respect for politics and politicians has never fully recovered from the expenses affair.

Kathryn Stone, the independent Parliamentary Commissioner for Standards, arguably does treat offenders harshly – she has to. Anyway, voters might well think that a 30-day suspension from Parliament, which was Mr Paterson’s penalty, is a pretty light rap on the knuckles. He was a fool not to have just accepted it. Now, following his resignation, he is suspended from Parliament for life.

The former Northern Ireland Secretary insisted that his lobbying was for genuine reasons of public concern.

One of the firms that paid him, Randox, had discovered health risks to do with the use of antibiotics in supermarket milk. He notified the Food Standards Agency about this, which was within the rules. What wasn’t was his subsequent attempts to promote Randox in the most “egregious” manner, according to the report from the parliamentary standards committee. He repeatedly approached ministers and officials on behalf of Randox and another firm, Lynn’s Country Foods.

This is so redolent of the 1990s that I was surprised at Mr Paterson’s sheer brash neck. I sat through the Nolan Committee hearings on sleaze in 1994/5 when I was a member of the parliamentary journalist lobby. I recall MPs explaining to me, straight-faced, that their paid lobbying actually benefited politics. They said it brought into Parliament the views of “the real world” of business. The presumption then was that no-one could actually live on their parliamentary salaries and they were actually doing a public service.

Some MPs saw themselves as lawyers – available for hire to anyone with the money. And it wasn’t only Tories. The former Labour transport secretary, Stephen Byers, was actually recorded telling a fake client that he was available as “a sort of cab for hire”.

But it was the Tories who raised lobbying to an art form with “cash for questions”. MPs were revealed to be taking up to £2,000 a time to ask questions in the House of Commons. The MP Neil Hamilton was accused by the Harrods boss, Mohamed al-Fayed of receiving his cash in brown envelopes. The MP for Tatton and his wife also enjoyed free holidays at the Paris Ritz.

To calm the fires, John Major set up the Nolan inquiry. t was too late to save his government, but the Committee on Standards in Public Life is still going strong. Its chairman, Lord Evans, lost no time in condemning Mr Paterson’s lobbying.

Lord Nolan begat the Seven Principles of Public Life, to apply to all public servants. These are: selflessness, integrity, objectivity, accountability, openness, honesty and leadership. It hardly needs to be said that the present administration would have camel-through-needle difficulties on all but the seventh. Boris Johnson certainly has leadership ability, at least as far as winning elections is concerned. But he’ll pass on honesty, integrity and the rest.

This has been a pivotal week for Boris Johnson – the moment the joke stopped being funny. He has been exposed not just as a risk-taker but a rule-breaker.

And with sleaze, the mud generally sticks.