Many limits are conventional rather than clear-cut, enforceable or institutional and therefore are subject to transgression.
One by one, the guardrails of American democracy are bending and cracking during Donald Trump’s Presidency. Whether the system can survive this historically unprecedented, and entirely self-inflicted, test remains unanswered. But there is a crucial object lesson already evident for all democratising societies, and any that seek to live by the rule of law.
In the latest, and in many ways most egregious, of his myriad transgressions of established political norms, Mr Trump on Friday commuted the sentence of one of his close associates, the notorious political fixer and self-proclaimed “dirty trickster” Roger Stone. Mr Stone was due to begin a lengthy prison term for lying under oath regarding his role as a conduit between Wikileaks and the 2016 Trump campaign.
He was not lying to protect himself. It is not unlawful for a private individual like Mr Stone to be in touch with an organisation like WikiLeaks and/or a presidential campaign. But it is highly illegal for a US presidential campaign to accept anything of value from foreign powers, including the huge dumps of hacked emails and documents from the opposition that WikiLeaks publicised, reportedly on behalf of Russian intelligence and in apparent coordination with Mr Stone.
Mr Trump testified under oath to the Robert Mueller investigation that he knew nothing of these communications, but other sworn testimony and most known facts, as well as common sense, suggest otherwise. But without Mr Stone’s corroboration, Mr Trump and his campaign were effectively protected from what would have been one of the biggest political scandals in US history.
Mr Stone has made no secret throughout that he believes Mr Trump owes him. And Mr Trump has all but acknowledged this, praising him, and his discredited, and also criminally convicted and Russia-linked former campaign manager, Paul Manafort, for not becoming “rats”, in Mafia lingo, by cooperating with authorities.
What is groundbreaking is not merely that Mr Trump is using these powers to reward friends, which other presidents, such as Bill Clinton, have allegedly done – although that is bad enough. It is to reward someone who, both sides basically admit, helped him cover up his own misdeeds to get elected.
It is as close as anyone can get to pardoning themselves.
And, indeed, the presidential pardon power, according to the US Constitution, does appear virtually absolute. The only open question is, could a president actually pardon himself? There are hypothetical legal arguments on both sides, because until now it has been considered a preposterous scenario.
No president, including Richard Nixon, has pardoned their own corrupt cronies like this or used pardon and commutation powers to facilitate or reward a self-serving cover up.
What has been revealed by the Trump era is how many of the guardrails of American democracy are conventional and cultural rather than clear-cut, enforceable and institutional. There are some important formal and structural limits, certainly, but much of what historically has appeared impossible or ridiculous turns out to be merely unconventional.
Mr Trump keeps demonstrating that US presidential power is more regulated by self-restraint than by constitutional or institutional limits, and that lesson clearly is not over.
Having overthrown a monarch, the framers of the Constitution were profoundly uneasy about creating a powerful chief executive in a centralised state. The first American political system, the ill-fated Articles of Confederation (1781 to 1789), was so decentralised that it could not function.
That is why the still-operative 1789 Constitution created a centralised federal government with the presidency as a powerful chief executive to make quick judgments about immediate necessities, particularly involving security and other urgent matters unsuited to large or even small committees.
Yet there was considerable anxiety about the potential for a rogue, monarchical executive, unmoored from self-restraint.
Proponents of the Constitution argued that filtering mechanisms such as the electoral college and the good judgment of the people and, especially, the elite, would prevent an unscrupulous demagogue from seizing this enormous power, and that the institutional constraints of Congress and the courts would intervene to prevent them from misusing it if that ever did happen.
Ironically, in the 2016 presidential election, the electoral college delivered the presidency to Mr Trump although Hillary Clinton beat him by almost three million votes in the popular tally. And Congress is now almost entirely guided by partisan interests, with the Republican Senate majority uninterested in the facts of the Ukraine scandal and declining to hear from a single witness before acquitting the President.
Many, and arguably all, presidents have played games with their vast powers. Nixon spied on and undermined his political enemies. Abraham Lincoln suspended civil liberties. Franklin Roosevelt rammed through his New Deal. Lyndon Johnson twisted arms and lied about Vietnam. George W Bush used cooked intelligence to invade Iraq. The list is endless.
But until now, the US has never had a president who does not acknowledge that self-imposed limitations are essential to the proper functioning of the office.
The Supreme Court – which remains a functional, independent institution largely because of the political savvy of Chief Justice John Roberts – last week slapped down Mr Trump’s claims of total immunity from criminal and congressional investigation, but put limits on what can be demanded of him.
If the courts move quickly, that could prove catastrophic for the President. But don’t hold your breath.
Instead, we are again reminded that the primary remaining, and always paramount, guardrail is the next election.
Mr Trump, deeply behind in the polls, now daily tweets the phrase “Rigged Election!!! 20% fraudulent ballots?” conjuring the spectre of millions of forged mail-in ballots, implicitly prepared by a foreign power, to defeat him.
We may see more untested guardrails rammed later this year, especially if events keep going badly for him.
If defeated, will Mr Trump leave office? If he tries to cling to power, will other institutions thwart him? Before he goes, will he try to actually pardon himself? Can he?
But the main lesson for the rest of the world is already clear. Conventional and cultural restraints are not worth much. Never say, “no one would do that”. Put it in writing, with consequences. Otherwise, it is a matter of time.