No election has a global impact to match a US presidential contest. Few White House races have been more bitter than the current one between Donald Trump and Hillary Clinton. So any public official who intervenes in it, 11 days before polling, needs to be very sure what they are doing. Such interventions don’t come bigger than the FBI director James Comey’s announcement that the bureau is reopening its probe of Mrs Clinton’s private emails. The potential impact is huge. Until last Friday, Mrs Clinton was looking on course to hand Mr Trump a decisive beating. It may still happen. But that is now all up in the air.

Mr Comey’s defence appears to be that justice must be done and that he would be damned if he made the announcement and damned if he didn’t. It is certainly true that US politics has become so polarised and so riddled with paranoia that the FBI director faced an unenviable choice. If he had carried out the new investigation behind closed doors and it had only become public after the election, whatever the result, one or both sides would be certain to cry foul. Conspiracy theorists would have had a field day. Yet by making it public now, he makes the investigation itself into a red-hot pre-election issue in a contest that will shape not just America but the world – and that’s awful.

This is precisely why the US Justice Department, with which the FBI works, has an established framework of rules designed to protect its impartiality from allegations of political bias. The department puts strict limits on disclosures about ongoing investigations and especially on anything that might be seen as influencing an election. A memorandum to this effect has been issued by a succession of US attorneys general every four years, including in 2016. Mr Comey has broken with this. Former Justice Department officials from both political parties have denounced him for this over the weekend. The current attorney general is believed to have advised against it, too.

Mr Comey has also compounded the potential abuse of power in two important ways. First, the investigation is at an embryonic stage. It may go no further. In his Friday announcement he spoke of emails “that appear to be pertinent” to the previous investigation he completed – concluding then that there was no case to answer – in the summer. But the emails come from a separate investigation, may not even be from Mrs Clinton at all and the FBI chief has not read them. His phrasing was extraordinarily vague. Mr Comey has thus launched his missile with minimum facts and maximum innuendo. There are echoes of British police blunders over child sex allegations. Even louder are the echoes of anti-Clinton fanatics like Judicial Watch. This strikes at the democratic process, not just individuals.

Second, Mr Comey should not have deferred to Congress, as he did last week. The investigation and prosecution processes should be kept rigorously apart from politics. That’s especially true at election time and even more necessary in such a polarised and acrimonious polity as the US Congress has become. One of the recipients of Mr Comey’s report, Jason Chaffetz, announced even before the FBI director’s move that he intends to begin oversight hearings into Mrs Clinton as soon as she is elected, if she is. Incredible though it may seem to the outside world and many Americans, impeachment efforts by Republicans against Mrs Clinton are already a serious possibility. Law enforcement officials have to be completely scrupulous in such a world. Mr Comey has not been. It is a step towards a lawless process.

Mrs Clinton is in many respects the author of her own problems on the email row. But Mr Comey took exactly the wrong decision. If the emails needed investigation, he should have authorised one behind closed doors, carried it out thoroughly, reported it to prosecutors and made an announcement, if one was necessary, in the fullness of time. This is called due process and it is a bulwark of the rule of law, not rule by politicians – least of all rule by a potential tyrant like Mr Trump.

Instead, Mr Comey has rocket-fuelled a venomous contest just when Mr Trump was desperate for a lifeline, just when Republican candidates in other races were craving something to energise their voters and just when some media were becoming bored with the predictability of the White House contest. His action was at best naive and at worst an abuse of office. It may, ultimately, have rather less effect on the election outcome than some suppose. Yet it is no consolation that Mr Comey has also guaranteed that, if no charges are brought, he will inevitably be accused of rigging the outcome.