A few months ago Harriet Harman caused outrage by suggesting that the “court of public opinion” mattered more than a court of law on City pay. Sir Fred Goodwin should, she said, waive part of his £650,000-a-year pension from the Royal Bank of Scotland to assuage people’s anger with bankers.

Her comments provoked a backlash. But Sir Fred did eventually shred his pension package. The Government stepped up the rhetoric over bonuses. And it is now widely assumed that the court of public opinion should take precedence over the letter of the law when it comes to MPs’ expenses.

This week Sir Christopher Kelly, the chairman of the Committee on Standards in Public Life, will publish his proposals for reforming the system. It won’t just be about duck houses and moats. He is expected to call for a ban on MPs employing their spouses and to suggest that those who live near London should not be allowed to claim for a second home. The worry is that the new rules will make it less appealing for poorer people and women to stand for Parliament.

Sir Thomas Legg has already dispensed some rough justice, applying rules on MPs’ expenses retrospectively in a way that could almost certainly be challenged, successfully in a court of law. His arbitrary ruling on the amount that MPs should have claimed for cleaners was bizarre. His decision that they would have to pay back tens of thousands of pounds that they were told they could claim by the Commons authorities was unjust.

But the court of public opinion, in which voters are both judge and jury, has already found the political class guilty of all charges. In the week of the anniversary of the gunpowder plot, most voters would like nothing better than to blow up the House of Commons, stoking the fire with tables and chairs purchased from the John Lewis list.

As confidence in political institutions collapses, and the media jeers the end of an elite, it is hard to find anyone who has the credibility to stand up for Parliament. An MP who said: “Let us claim for Kit Kats” would be like Marie Antoinette declaring: “Let them eat cake.” This raises an important question — is the court of public opinion a demonstration of democracy in action or an example of mob rule?

There is no doubt what most MPs think. The Tory backbencher David Wilshire complained in a letter to his constituents recently that MPs were being treated like Jews in Nazi Germany. “Branding a whole group of people as undesirables led to Hitler’s gas chambers,” he wrote.

Although most MPs would not go this far, the anger at Westminster is palpable. “We’re all being punished for the misdemeanours of a few,” one backbencher said. A minister added: “My colleagues are obsessed by the expenses issue. There’s so much fury they can’t think about anything else and a lot of MPs are in denial about how bad this is for all of us.”

Certainly there is nothing rational about the court of public opinion. The country should be governed according to the rule of law, and through democratic elections, rather than by a political version of The X Factor in which the voters deliver their verdict week by week.

Britain is not like Afghanistan, where corruption means ballot boxes being stuffed to rig an election. But politics is about mood and emotion as well as reason and for the rule of law to be upheld the system has to command respect.

As Shakespeare observes in Measure for Measure, a study of the relationship between justice and mercy: “Man, proud man [is] Drest in a little brief authority.” Once the authority slips completely anarchy looms — or the voters will look for an alternative source of leadership, such as the BNP. The court of public opinion does matter politically because without it the rule of law cannot work. Denied the opportunity to deliver its verdict, through a general election, the jury will only grow angrier. “There needs to be some blood letting,” admitted a minister. “The public has to see sacrifice if we are going to lance the boil and get rid of the poison.”

This is not just about expenses. Professor David Nutt was sacked as the Government’s chief adviser on drugs because Alan Johnson did not believe that the court of public opinion would support his suggestion that taking Ecstasy was no more dangerous than riding a horse.

The Home Secretary was right to say that the classification of drugs is a political, rather than a scientific, issue — but partly because of the wider loss of trust in MPs he did not feel able to have an adviser with whom he disagreed. The man in the white coat, appointed to give credibility to a discredited political class, ended up further undermining the authority of the Government. Similarly, on Iraq, Afghanistan and climate change, ministers have been like barristers unable to convince the jury to support their positions.

Politicians cannot ignore the court of public opinion but they have to try to harness it to prevent it turning into a lynch mob. They cannot just try to gauge its views through focus groups or attempt to give the impression of listening by promising more direct democracy — the chief justice of the California supreme court recently criticised the state’s widespread use of referendums, saying it had “rendered our state government dysfunctional”.

Sir Ian Blair, the former Metropolitan Police Commissioner, is right to raise concerns about Conservative plans for elected police chiefs — that will simply encourage knee-jerk populism. Politicians have to engage with the voters and persuade them.

The real problem about expenses is that they have made it harder for politicians to show leadership about the things that matter far more. The verdict of the court of public opinion is too harsh on many MPs. But unless they accept it, serve the sentence and move on, they will never be able to convince the voters to listen to them on anything else.

Times Recommends

• Care in old age: we won’t pay, the State can’t
• Viewers’ choice: our talentless Everyman
• Memo to the poor: go forth and don’t multiply