The assumption that Hillary Clinton would win the 2016 presidential election influenced fired FBI Director James Comey's decision to make a last-minute announcement about the investigation into her use of a private email server at the State Department, Comey told ABC News in an interview airing Sunday night.

ABC News' George Stephanopoulos asked if Comey's assumption that Clinton would win the presidency in 2016 was a factor in his decision to send a letter to Congress in October claiming additional emails had been discovered related to the email investigation, days before the election. Comey said it, "must have been." Clinton has said she partly blames Comey's handling of the investigation for her loss, as do many of her supporters.

"It must have been," Comey told Stephanopoulos. "I don't remember consciously thinking about that, but it must have been. Because I was operating in a world where Hillary Clinton was going to beat Donald Trump. And so I'm sure that it was a factor. Like I said, I don't remember spelling it out, but it had to have been, that she's going to be elected president and if I hide this from the American people, she'll be illegitimate the moment she's elected, the moment this comes out."

Stephanopoulos asked if Comey knew that letter would elect Mr. Trump, if he would still send it.

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"I would," Comey said.

"If I ever start considering who's political fortunes would be affected by a decision, we're done," Comey added, referencing the FBI. "We're no longer that group in America that is a part from the partisans and that could be trusted. We're just another player in the tribal battle."

On Sunday, Mr. Trump appeared to respond to Comey's comments, tweeting that the former FBI director was a "slimeball" for "making decisions based on the fact that he thought she was going to win, and he wanted a job."

Unbelievably, James Comey states that Polls, where Crooked Hillary was leading, were a factor in the handling (stupidly) of the Clinton Email probe. In other words, he was making decisions based on the fact that he thought she was going to win, and he wanted a job. Slimeball! — Donald J. Trump (@realDonaldTrump) April 15, 2018

Comey's new book, "A Higher Loyalty," will be released on Tuesday, although multiple news outlets, including CBS News, have obtained copies of the book ahead of time. In the book, Comey describes Mr. Trump as someone "untethered to truth." Mr. Trump had responded to the book, calling Comey a "slime ball" and saying he 'should be prosecuted."

In March 2017, Comey confirmed the FBI was investigating Russian interference in the 2016 election. Mr. Trump fired Comey in May, leading to Deputy Attorney General Rod Rosenstein to announce the appointment of special counsel Robert Mueller.