In his new book, A Higher Loyalty, ex-FBI director says he rejected lawyer’s concern about investigation’s potential political impact

James Comey’s confidence that Hillary Clinton would win the presidency informed his decision to notify Congress just 11 days before election day that an investigation of her emails had been reopened – an act many, including Clinton, believe instead helped usher Donald Trump into the White House.

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In Comey’s new book, A Higher Loyalty, a copy of which was obtained by the Guardian on Thursday, the former FBI director writes that before he wrote to Congress to say investigators were reviewing a newly acquired batch of Clinton emails, a “brilliant and quiet” lawyer on his team asked him a pointed question.

“Should you consider that what you are about to do may help elect Donald Trump president?”

Comey thanked the lawyer for her question, he writes. And then he answered with a resounding: “No.”

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What Comey did next – following a July news conference in which he branded Clinton’s handling of classified material “extremely careless” – has left him vulnerable to accusations of having swayed the election. News of his letter to Congress splashed in the headlines as early voting proceeded.

Comey writes: “Assuming, as nearly everyone did, that Hillary Clinton would be elected president of the United States in less than two weeks, what would happen to the FBI, the justice department or her own presidency if it later was revealed, after the fact, that she still was the subject of an FBI investigation?”

In addition to its evisceration of Trump – who is repeatedly likened to a mafia boss and labelled “unethical, and untethered to the truth and institutional values” – A Higher Loyalty offers a painstaking recounting of the Clinton emails saga, which Comey calls “this awful case”. While stopping short of regret, Comey does admit faults in his conduct of the affair.

“Hindsight is always helpful, and if I had to do it over again, I would do some things differently,” he writes.

Clinton has said Comey “forever changed history” with his election interventions. Comey has testified that it makes him “mildly nauseous to think we might have had some impact on the election”.

Yet in his book, Comey admits that at the July 2016 news conference, in which he spoke for more than 10 minutes about Clinton’s poor email practices before announcing that the FBI would not recommend that she be prosecuted, he might have buried the lede.

“I would avoid the ‘Seacresting’ mistake by saying at the beginning of my statement that we weren’t recommending charges,” he writes. “At the time, I thought there was a risk people wouldn’t listen carefully after the headline, but looking back, the risk of confusion from me delaying the conclusion was greater.”

He continues: “More important, I would have tried to find a better way to describe Secretary Clinton’s conduct ... my use of ‘extremely careless’ naturally sounded to many ears like the statutory language – ‘grossly negligent’ – even though thoughtful lawyers could see why it wasn’t the same.”

Play Video 3:23 Trump and Comey’s hate-hate relationship – video explainer

Comey is frank about what a distasteful task the investigation of a major presidential candidate represented for an agency that aspired to preserve its independence in an election rent by poisonous partisanship.

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“I knew this was going to suck for me” to hold the summer press conference, he writes. “From the Democratic side would come predictable stuff about my wanting the spotlight, being out of control, driven by ego. From the Republican side would come more allegations of justice department incompetence or corruption.”

As palpably unhappy as he is with the Trump presidency, Comey’s soul-searching about his election conduct does not culminate in a dream of a do-over.

“But I believed – and still believe, even in hindsight – it was the best thing for the FBI and for the Department of Justice” to go public about Clinton, he writes.

“The American people needed and deserved transparency,” he says, adding that he believed he had the “independent reputation” to “take the hits” that would protect public trust in the FBI.