Transcript for Chris Christie on his reason for endorsing Donald Trump during the campaign

Texas, I'm here today to proudly say I endorse Donald Trump for president of the United States. America must have a strong leader again that can restore American jobs, that can restore American confidence, and Donald Trump is just the man to do it. That was former New Jersey governor, Chris Christie, and 2016 presidential candidate, Chris Christie making his surprising endorsement of his long-time friend. He's written a new book about his rise to political power and time as a white house insider called "Let me finish," and I promise we will try to do just that because we got a lot to talk about. Please welcome Chris Christie. Hello, governor. Hi. We've gotten to know each other here at ABC because we're both ABC political contributors. Before I got to know you personally I remember this when this happened because you were the first mainstream politician to endorse Donald Trump. I remember when you did it. I actually was still dating my husband. I went on a double date with two other people who work in Republican politics and I said this is the beginning of him getting really normalized because when Chris Christie is endorsing him this makes him a mainstream candidate and we cheersed and said this may be the beginning of the end. You had a quick change of heart. What made you make that decision? He was going to win. You understand the practicality of politics. I had been running against him and trying to beat him for 6, 8 months, however long it was. He came in second in Iowa by a whisker. He won New Hampshire two to one and then he won South Carolina by double digits. If it was any other candidate in any other year, the media would have all said it's over. If you win in South Carolina, New Hampshire and you barely lose in Iowa, you're going to be the nominee. So my point was, all right, he's going to be the nominee, so let's get moving to try to get him to be better than he is. And that was the real reason I did it -- How is that working out? Well, you know what, joy, you don't know what would have happened if I hadn't, right, so you might have been even more sad than you are now. The fact is that I had known him for a long time. I felt like I had an ability to be able to talk to him in a way that others didn't and I think there were a lot of things over the course of time that I got him to either start doing or stop doing that were helpful. But in the end I was convinced at that point -- and as you know, a lot of other people in the party weren't and felt like, oh, no, this guy can beat him, Marco, Ted can beat him. My wife was doing door to door to me in New Hampshire. She said she was my wife. They said we like your husband, he's so smart and articulate. She said we're voting for trump. She said wait a second, if you love him why are you voting for trump, she said we don't need any more politicians. I felt like we better get behind this guy or we're going to lose in November and we better make sure that we try to make him the best that he can possibly be. You said he was exactly what the country needed then. Do you still feel the same way now? Well listen, I don't think anybody is -- first of all, that's political hyperbole. That's what you say when you try to endorse somebody. I've spoken many times about things that he's done, did in the campaign, and has done in the presidency that I don't agree with. Like what? Listen, there's plenty of things. The travel ban, the separation of families at the border. I don't agree with the wall. The shutdown. And the shutdown too which I just talked about recently. I mean, so there's plenty of things that he's done that if I were president I would do significantly differently, but there are also things he's done that I think have been good. So what I try to do -- Curious, like what? Okay. I don't mean to push it but -- That's good. I appreciate you giving me the opportunity to point out the good, joy. You're doing great. I believe that the tax cuts he's done are the right thing. I believe the deregulation he's done in Maine is the right thing. I like many if not most of the judges that he's put on the court and I think he's given us a better trade deal with Mexico and Canada than we had before and I think it's going to be good for our economy. Now, I would have gone about it differently than he did. I wouldn't have put tariffs on our friends. China I'm okay with because they're stealing from us blind all the time. But the folks that are our friends I wouldn't have done it but those are stylistic differences. In the end it's a better deal for us. How do you feel about Russia? In general, Russia is an adversarial country. Putin is a dictator. Are you comfortable with that relationship? No. It's not the way I would handle it. I think you know it. It's not the way I would handle it. I would be -- Sometimes you have to put country before party, right? I think you always have to put country before party, right? But -- But part of putting country before party is that people in this country all have different opinions about what that means. Somebody would say, well, I'm not going to be for Donald Trump because I'm putting country before party. Well, maybe I agree more with him than I agree with Hillary Clinton and that was the truth. That's the other thing about elections. Elections are binary choices. We don't vote for who we want to vote for. We get to vote for who's there. So Hillary Clinton and Donald Trump were the two most unpopular general election candidates in the last 60 years. So a lot of people went in and voted the opposite. But I have a question that was -- it just struck me in reading your book. Everyone knew about what was happening for you personally with bridge-gate. Right. It was -- and you've been friends with trump for 16 years and we understand that. I'm just surprised that friendship survived the primaries because in your book you say when you're going through bridge-gate your son comes home from princeton and says to you, I've got to ask you something, I've got to look you in your eye and say did you do it? Yep. As a mom, it seemed to me that you looked at him and you write, of course not. And at that moment you knew you had to speak out. And then you say later on during the primary, Donald Trump says at a December 7th rally -- In South Carolina. In South Carolina. The George Washington bridge, he knew about it. They're closing up the largest bridge in the world. He knew about it. He knew about it, totally knew about it. And then you write Donald's attacks were unfounded, and worse, he knew it. He had told me on so many occasions. He almost brought you down. This bridge-gate almost brought you down as governor. Almost threw you off the bridge. He lied about it. Right. And then you get on national television and stump for him. Right. Okay, so -- How do you reconcile that within yourself? Because in my world those are shots fired. You call me a criminal I'm not stumping. Let him answer the question. All right, yeah those were shots fired. Yeah. And in your world maybe when shots get fired you have to go to, as they were say in the mafia, to the mattresses and everybody just tries to kill everybody. In politics shots are fired every day, and the people who fire a shot at you today are people that you have to work with tomorrow. This is what confuses me at times and I'll get to a direct answer but I think there's a bigger point too. Republicans and Democrats shoot at each other every day. They have different points of view and they shoot at each other. We say we want bipartisanship. You got to be willing to take incoming shots and say, okay, that's what they think, that's what they do, now let me see if I can figure out a way to work with them. He did that in the afternoon. I was in Iowa. I got the reports about what he said. I went out that evening and I went after him hard, really hard, harder than I had ever gone after him in the campaign. What happened the next morning? His campaign manager calls my campaign manager and says, Donald made a mistake, he should have never said that stuff, he knows it was wrong, he would like a truce. Why did he call you? Abby I'll get to that. It's only an hour show though. I understand. I said to my campaign manager, if he wants a truce, he calls me. Later that afternoon he called me and he said I was wrong, I was angry. What was he angry about? He was angry about the fact that I had gotten the Manchester union endorsement right before that. He wanted it, I got it. He was pissed an off he went. Can I tell you, he's never given me or my family an apology for anything and -- He doesn't do that. Granted, I think what was said about my father was worse. It was. By the way, he did apologize to me so when he apologized to me and never said that stuff again, you have a decision to make. Do you hold a judge and say, he said it once, it makes him an awful person, I'm not going to deal with him. If he do that in politics with every person who dwoosgoing to be eating dinner alone. We are just going to take a break.

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