The Clinton campaign is hinting at skulduggery after the bureau publicly declared it was reopening its emails inquiry

Democratic party outrage at the FBI’s shock revelation, almost on the eve of the US presidential election, that it is resuming its controversial investigation of Hillary Clinton after declaring in July that the matter was closed, has been fierce.

Donna Brazile, chairwoman of the Democratic national committee, was one of many to cry foul. “The FBI has a solemn obligation to remain neutral in political matters. Even the faintest appearance of using the agency’s power to influence our election is deeply troubling,” she said.

John Podesta, the Clinton campaign chief, also hinted at skulduggery: “It is extraordinary that we would see something like this just 11 days out from a presidential election.” Podesta said the FBI statement left it unclear whether there was anything remotely new to investigate.

This may well be true. In his letter of notification to Congress, James Comey, the FBI director, said he did not know whether a batch of newly discovered emails bore any relation to previous claims that Clinton acted illegally in sending private emails containing classified information when she was secretary of state from 2009-13.

Comey said the new emails “appear to be pertinent”, but he and his officials had not yet examined them. In a later statement, he admitted “we don’t know the significance” of the emails, but said he felt obliged to investigate and make his decision public. It is not even certain that the emails came from Clinton’s private server.

Comey’s move is either extremely naive or extremely cynical. FBI investigations are routinely conducted behind closed doors. Only when a decision to prosecute has been taken, based on persuasive evidence, is an inquiry made public or suspects named.

For a respected, ostensibly independent figure like the FBI director to tip his hand at this extraordinarily sensitive moment amounts to an overtly political, partisan act. Since it must be assumed that Comey is no fool, it must also be assumed that he knew what he was doing.

A less damning explanation is that he was clumsily attempting to save the FBI (and himself) further criticism from the Republican right, which denounced his earlier investigatory efforts as a pro-Hillary cover-up. But if that is the case, why did Comey not have a confidential word with the relevant congressional oversight committees? Nobody could then subsequently accuse him of a cover-up. And he would not have triggered the firestorm in which the FBI’s impartiality is again being questioned, and this time from the left.

Whatever Comey’s reasons, the damage is done. Clinton must now try to weather the storm until 8 November, having previously believed that she was in the clear. It will be a rough ride. The ugliest and most personally vindictive presidential campaign in modern history is now set for a frenetic, furious and nail-biting finale.

Donald Trump and his supporters have been quick to wade in. Speaking on the campaign trail, the Republican nominee revived jibes that Clinton is a corrupt and habitual liar. In typically hyperbolic terms, he said that her conduct was “the biggest scandal since Watergate”.

As usual, Trump’s risible exaggeration undermines his case. But the genuine anger and alarm expressed by Clinton’s surrogates reflect their fear his attacks may be hitting home and their knowledge of how damaging renewed questions about her honesty and competence could potentially be.

Although in Trump she faces the least qualified and, for a host of reasons, most objectionable Republican candidate in living memory, Clinton is still only about 5% ahead in the polls, as a national average.

This bald figure may be considered misleading. In 14 battleground or “swing” states, where the electoral college will be decided, Clinton is ahead in all but Ohio, Iowa and Georgia. But polls across the nation are tightening. This latest flurry of negative headlines in national media desperate for a sensational story to keep the race alive (or in some cases, to dish her) will not help.

Despite her advantages in experience, name recognition, political achievement and spending power, Clinton has evidently failed to knock Trump out of the race.

Despite a deeply damaging few weeks for Trump, when he was beset by allegations of sexual misconduct, he still appears, amazingly, to be in with a shot.

A more skilled and savvy politician than Clinton could and would have buried Trump months ago. A more charismatic, popular or likeable Democratic standard-bearer could now be awaiting electoral coronation. This latest blow may cause some underwhelmed Democrats to abstain. And Clinton’s best pitch to sceptical independent and undecided voters – that she is the least worst candidate – leaves her vulnerable to a last-gasp upset.