The decision of Donald Trump to fire the person overseeing an investigation into him is a turning point in the tragedy of the United States’ 45th presidency. At least when Richard Nixon did so, it was a blatant – and doomed – attempt to save his presidency. Mr Trump says he sacked his FBI director, James Comey, because he was not doing a “good job” and that he had been unfair to Hillary Clinton over claims of misuse of a private email server. This is not remotely credible: Mr Trump spent last year’s campaign saying Mrs Clinton should be locked up for such carelessness, and kept Mr Comey in place for four months after taking office. No facts have changed. Instead Mr Trump has meddled in a federal investigation, which by all accounts was expanding rather than shrinking, into plausible claims that he owes his office to the clandestine intervention by a hostile foreign spy service.

Mr Trump’s act is a threat to US governance. It looks ominously part of a pattern of trespassing beyond constitutional boundaries. Mr Trump fires officials who cross him. He attacks judges when they find his policies unlawful. He refuses to release his tax returns, which might reveal conflicts of interest. He uses blind trusts that are not blind, while his children comingle private and public business. Power in the Trump presidency is held by the president’s family, and what appears to be incompetent followers whose main contribution is loyalty rather than expertise. In claiming that Mr Comey had given him three private assurances that he wasn’t under investigation, Mr Trump broke a protocol long-observed: that presidents do not publicly comment on an ongoing investigation. Especially one that centres on them. In short, Mr Trump is governing like he is the president of a banana republic, not the leader of the oldest constitutional government of modern times.

There is a respectable argument that the wheels of democracy are turning in the US. Four congressional committees are investigating potential collusion between Mr Trump’s associates and the Russian government. But events reveal an obstructive and uncooperative White House. Mr Trump’s officials, last month, refused a congressional request for documents related to Michael Flynn, the former national security adviser who it is now claimed was “compromised” and open to blackmail by the Russian government. Such behaviour gives the impression of guilt.

Although Mr Trump’s Republican party has majorities in both US legislative chambers, he appears to alienate Congress, and his ambitions on tax cuts, healthcare and other key areas look increasingly unachievable. It is a positive sign that a handful of Republican senators are questioning the timing of Mr Comey’s firing. The manner of the FBI boss’s removal also means that an independent special prosecutor is now needed to investigate the Russian influence on the elections. Since Mr Trump’s attorney general has recused himself from any Russian investigation because of links to Moscow, his deputy would be responsible for overseeing the FBI on this matter. Yet his deputy provided the evidence for Mr Comey’s dismissal. That is why only an independent counsel will serve.

The US system of checks and balances is a metaphor, not a mechanism. There is a separation of powers and judicial independence. But it relies on the right people making the right choices at the right time. A US president is restrained largely by conscience, training and the desire to be judged well by history. Mr Trump appears to lack those qualities. Senators have to approve Mr Trump’s new FBI director. They should only do so if Mr Trump’s White House appoints a special prosecutor. They may prefer to keep quiet, opting not to rock the boat. But that will let presidential tyranny take hold. It won’t occur instantly. But it will, like dusk, draw in without us realising.