T he charges against Donald Trump in his impeachment trial boil down to this: he tried to cheat his way to a second term, got caught, and attempted a cover-up. The difficulty is that Trump, notwithstanding heaps of damning factual evidence of deliberate wrongdoing, is 99% certain to get off.

Yet the trial in the Senate cannot be dismissed simply because the result is known in advance. Its perverse outcome, if confirmed, may seriously undermine respect for American democracy and the constitutionally based rule of law. That’s bad news for the world at large.

Take the legal aspects first. American observers of Britain’s Brexit travails had great fun ridiculing the archaic rules and conventions that for months tied parliament up in knots. Yet the US situation is arguably worse.

The impeachment process, contingent on proof of “high crimes and misdemeanors”, is a game played under 18th-century English rules, though not English spelling, that even American legal scholars struggle to explain. Trump is charged, firstly, with abuse of power. The framers of the constitution “were particularly concerned that a sitting president would abuse his office to get re-elected,” said Noah Feldman, a Harvard law professor, in a recent analysis.

According to the “English tradition” followed by the likes of James Madison and Alexander Hamilton, Feldman said, impeachable offences occurred “when a high office-holder took gifts of value to do his job”.

In Trump’s case, the alleged gift of value was Ukraine’s looked-for agreement to investigate his Democratic rival, Joe Biden, and his son, for evidence of corruption.

Yet the president’s lawyers reject the basic premise. They say Trump was fulfilling his duty to root out corruption in Ukraine, a recipient of US aid. There was no “quid pro quo”. What Trump did, they argue, cannot be defined as a high crime or misdemeanour – whatever they may be.

Republican sycophancy means the Senate often more closely resembles Russia’s rubber-stamp Duma or Iran’s Majlis

Trump’s defence goes much further. By denouncing the entire process as a hoax and refusing to cooperate – which gave rise to a second charge of obstruction – he has, de facto, denied Congress’s right to impeach him at all. The very idea an accused person could take such a stand was never envisaged by delegates to the 1787 constitutional convention in Philadelphia and is wholly unprecedented.

“A president who cannot be criminally investigated [due to the immunity traditionally afforded an incumbent] and also cannot be investigated by Congress would be effectively above the law … Denying Congress’s power to conduct an impeachment inquiry subverts the foundation of democratic government,” Feldman wrote.

This reliance on time-worn, disputed English legal precedents has exposed a dangerous US constitutional weakness, as Adam Schiff, the House of Representatives’ “lead prosecutor”, recognised last week. “I don’t think the impeachment power is a relic. If it is a relic, I wonder how much longer our republic can succeed,” he said.

“The president has shown he believes he’s above the law and scornful of constraint … If we don’t stand up to this peril today, we will write the history of our decline with our own hand… Our future is not assured.”

Political aspects of the trial are no less threatening for US democracy. The Senate was conceived as a body of independent, high-minded individuals acting in the national interest. Today it is anything but, dominated by a rabidly partisan Republican majority determined to acquit Trump no matter what.

Quite why Chief Justice John Roberts, who presides at the trial, calls the Senate the “world’s greatest deliberative body” is puzzling. Republican sycophancy, fed by fear of Trump, means it often more closely resembles Russia’s rubber-stamp Duma or Iran’s Majlis.

Roberts’s supreme court provides more evidence of a fundamental breakdown in the fabled US constitutional system of checks and balances. Trump has successfully nominated two conservative justices to the court – Neil Gorsuch and Brett Kavanaugh – and more may follow. He is also shamelessly packing the federal bench. Public trust in the independence and open-mindedness of the judiciary is plainly at risk.

It’s entirely possible the electoral college, another 18th-century remnant, will deliver the presidency to Trump in November, contradicting the popular vote (as happened in 2016 and 2000) and, for many, rendering the election meaningless.

Unchecked, underhand voter suppression, gerrymandering of electoral districts, and antiquated and insecure voting equipment all fuel doubts that the US is still capable of holding free and fair elections.

The chronic failure to curb a system of money politics that favours incumbents further exacerbates democracy’s credibility problem. So, too, does the politicisation of offices of state such as attorney-general – where the present incumbent, William Barr, acts more like Trump’s consigliere than chief government law officer.

Factor in digital meddling by foreign powers, notably Russia, and Trump’s “fake news” culture of lies and manipulation, and a sobering picture takes shape of a democratic system on its knees and close to possible collapse.

For a watching world where democracy is widely under attack from autocrats and populists and where the US claims to set an example, this is a chilling spectacle.

Of all the many wrong things Trump has done, his most terrible legacy may be the destruction of trust in the workings of democracy, the US constitution, and the rule of law. His weird, slightly unreal trial in absentia symbolises that dread prospect.

E pluribus unum – out of many, one. And that one a tyrant, just as the founding fathers feared.