Transcript for Comey sits down for first one-on-one interview since he was fired

Assad not to gas his people again. S, thanks so much. Next, to a new excerpt from that blockbuster book by James Comey. Tting downith George Stephanopoulos. Tonight, Comey R new S from one of the most controversial choices of his career, why he went public reopenine investigation into Hillary Clinton just days before the election here's abcavid Wright. Reporter: It was the October surprise thallary Clinton torpedoed her chance to be president. Tonight, Brea news, the FBI bombshell for Hillary Clinton. Reporter: Just ten days before T action, thefbi directoresent a letter to congress, alerting lawmakers the FBI found new e-mails "That appeared to be peen a compute Clinton's top aide huma abedin shared with her husband, disgraced fo C anony wiener Hillary con's convinced that ttter defeated her, what do you say to her? I Hnot. I don'ow. I honestly don't know. I sure hope Reporter: ABC's George Stephanopoulos pressed Comey about that. Tryrealize that I'm tryin' to help a candidate or rt a candidate, I'm tryin' to do the R thing. Reporter: And while Comey doesn't admit he handed the prescy to trump, hes say he assumed, at the time, that Clinton Wakely to win. I don't remember spelling it out, but it had to haven. That -- that she's gonna be eld president, and if I hide this froe ICAN people she'll be illegitimate the moment she's elected. Reporter: At the time, candidate P applauded Comey's decision. And it took guts for ctor to make the move the made. Reporter: Now pdent trump calls Comey "A weak and untruthful slime ball." Quote, "His handling of crooked llary Clinton case, and the events surrounding it, willo down as E of the worst 'botch jobs' of history." Comeyl be forev known as a disgracartisan hack at broke his sacred trust with thesident of T United States. EP Comey tells George he's not sure what other choice he had. As I in the book, I -- think I did it the what it shouaven done. I'm -- I'm not certain of that. Other people migave had a different view. Pray to god, ntubi director ever has to find out. Reporter: Com book comes out Tuesday, it'e first big memoir from a majoayer in the trump white house. Someone who's bee the lightning rod of so much controversy. The itself and geor interview sparking new debate about trumd about Comey, easily the most controversial FBI chief since J. Edgar Hoover. Tom. We all lrward to that interview. David, thanks so much. Gege Stephanopoulos will have much more of his exclusive interview James Comey and the fierce reaction to it tomorrow morn "This we" and of course the full interview will aornight at 100 P.M. Eastern.

This transcript has been automatically generated and may not be 100% accurate.