Donald Trump is giving America and the world a lesson in the value of shame – and the power of shamelessness. Through his actions, most vividly his firing on Tuesday of the FBI director James Comey and his comments about it since, Trump has taught us that shame performs a vital democratic function – and how dangerous is the man who feels none of it.

First, though, take a step back. For years, reformers in this country and many others believed a written constitution was the true guarantor of democracy. Only such a document could properly entrench the rights of citizens and impose restraints on their masters. An unwritten constitution, like Britain’s, was no constitution at all, just a murky accumulation of custom and convention that surely had no force because it existed only in the ether: it wasn’t worth the paper it wasn’t written on.

I confess I have always belonged in that camp. Nearly two decades ago, I wrote a polemic, Bring Home the Revolution, urging Britons to adopt a written constitution, taking as my inspiration a text that, in its carefully weighted machinery and delicate construction of checks and balances was, to me, as beautiful as the inner workings of an antique clock: the constitution of the United States. By his actions this week, and indeed since he emerged as a presidential candidate, Trump is testing my admiration for that document – testing it, perhaps, to destruction.

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Take what he has done this week alone. In firing Comey, he was clearly breaching the wall that’s meant to separate law enforcement from political meddling. But he was not violating the constitution. Technically, a president does have the power to sack an FBI director. It’s just that the unwritten rules have always said a president shouldn’t. Trump saw those unwritten rules and walked all over them.

That’s how he operates. On Friday morning, Trump mused on Twitter that perhaps he should abandon the daily press briefing, long seen as an essential requirement of basic transparency. It’s not in the constitution, but every previous administration has regarded it as fundamental democratic practice.

The same is true of naked profiteering from public office. Past presidents have divested themselves of any business ties, or at least placed their holdings in a blind trust, lest there be even a hint of a conflict of interest. Not Trump. He says the law is on his side and “the president can’t have a conflict of interest”. Legal experts say that, strictly speaking, he might be right. But in the past, the strict technicalities were not the point. The unwritten norm was clear: no president should have his or her judgment clouded by the prospect of personal financial gain.

A similar rule has applied to the nepotistic hiring of unqualified relatives: not done. Trump has done it anyway. He’s appointed his daughter Ivanka to a senior, if vague, White House role while putting his son-in-law, Jared Kushner, in charge of such footling matters as restructuring the US government and negotiating Middle East peace.

On and on it goes. Convention dictates that, once in office, a president has a particular responsibility to speak carefully and honestly: the word of the White House should be its bond. Whatever looseness might be allowed a candidate is supposed to disappear once the oath is sworn: you can campaign in fibs, but must govern in truth. Of course, Trump’s predecessors have, more than once, famously fallen below that high standard. But they always strained and stretched to at least seem truthful. They made an effort, however contorted, to avoid an overt lie. That was the norm.

Trump has broken that one too. On Friday he tweeted a near-admission that his spokespeople lie but, hey, what can you expect, the White House is a busy place. His exact words: “As a very active president with lots of things happening, it is not possible for my surrogates to stand at podium with perfect accuracy!” That came after he had given an interview about the Comey firing that wholly contradicted the account his team had supplied until that moment. The pattern is clear. Conventions and norms that Washington had seen as inviolate – from the disclosure of tax returns to the acceptance of the integrity of national elections – Trump has been only too happy to violate. And he can do so without obvious consequence.

There are two key conclusions to draw. The first is that even the US, a society organised around reverence for its written constitution, also has an unwritten constitution, a set of norms so absorbed into the political bedrock they were scarcely noticed – until they were broken. It means that even an exemplary written constitution cannot alone protect a country from a determined assault on its democracy.

Which brings us to shame. The usual constraint that has held back past presidents, preventing them from violating democratic norms even when they had the power to do so, was a degree of shame. They would be ashamed to demonstrate naked greed, say, by lining their pockets or hiring their relatives from the public purse. To be caught out in a lie was to face political mortification. When they acted, Trump’s predecessors were not only thinking of what was allowed under the law. They also had an eye on their reputation.

Trump is different. He is a man without shame. Throughout his career in New York real estate, he showed a constant incapacity for embarrassment. Any move that might bring him more money or, now, more power, he will make – regardless of the ethics or the optics or the damage to his reputation. He is shameless.

What, then, of those actions by Trump that don’t simply violate an unspoken norm, or rely on a self-censoring sense of shame, but break the law? Surely Trump can be brought to account over those?

There is no shortage of such deeds. In the last 24 hours or so, he has provided evidence of two more. First, he told an NBC interviewer that, despite the version spun by his aides, his motive for firing Comey related to the FBI’s investigation into collusion between his campaign and Russia. That is a clear admission of obstruction of justice. On Friday morning, he tweeted a threat to Comey who, he suggested, had “better hope that there are no ‘tapes’ of our conversations before he starts leaking to the press!”. That’s intimidation of a witness. Both would surely count as what the constitution calls “high crimes” and therefore grounds for impeachment.

But here’s the problem. The constitution is powerless unless there are people willing to enforce it. Today, that means Republicans in the House of Representatives. Only they have the power to bring impeachment proceedings against the president. Given what we already know, the constitution all but requires them to do it. But their loyalty to party means they refuse to do their duty. They too feel no shame.

This, I see now, is the weakness of any written constitution. It cannot extend its protective reach to those valuable customs and traditions that stand outside it. And it cannot even defend its own provisions when those with power refuse to honour their democratic obligations.

Even the most ancient and beautiful documents cannot protect us. Only the respect of the powerful for those documents can do that. When that is gone, when they feel no shame, democracy stands naked – and vulnerable.