Trump's niece calls on him to resign

Mary Trump speaks exclusively to ABC News about President Trump for the first time since the publication of her explosive new book.
6:03 | 07/15/20

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Transcript for Trump's niece calls on him to resign
First, my exclusive interview with president trump's niece Mary trump. She just published a scathing memoir of her uncle and family who went to court to prevent her from doing any interviews. I sat down with her after a judge ruled she could not be stopped from speaking out. It is a deep dive into her past with the trump family but Mary is very focused on what the country should do right now. What's the single most important thing you think the country needs to know about your uncle? He is utterly incapable of leading this country. And it's dangerous to allow him to do so. Based on what you see now or what you saw then? Based on what I've seen my entire adult life. What do you want to happen now? This country is on a precipice and we have a decision to make in the not too distant future about who we want to be and where we want to go as a country. It's hard for me to process just how many awful things are going on simultaneously on a daily basis, and people need to know. People need as much information as is available in order to make a decision that makes sense for our future. As a country. As Americans. As citizens of the world, so to speak. You do have one potentially explosive allegation in the book, at least one and you write that when the president was trying to transfer from Fordham to Penn, he had someone else, a man named Joe Shapiro take his S.A.T.S? Yes. This was 1964. How do you know that? I've been told this by people in my family. I am absolutely confident that it's true. I'm happy to finally be able to speak about it. I also know that it was not the Joe Shapiro people have been focusing on. It's to be clear it's not Pam shriver -- No. Pam shriver, the widow of Joe Shapiro said he didn't meet Donald Trump until Penn. There's no way this is true. And I feel terrible she's been subjected to this. Honestly I wish I could have said something sooner and I think the only people other than me who can address it are other people in my family and look forward to hearing their response to that question. How do you know it's true? I trust my sources. Is Joe Shapiro still alive? That, I don't know. I have no idea. Have you ever met Joe Shapiro? I have not. So you believe this your sources. Uh-huh. How do your sources know? They were alive at the time, so they have firsthand knowledge of this. And you believe other members of your family also know this is true ask. Yes. White house says it's absurd falsehood. Of course. They would. Can you prove it's true? Can I prove -- no, because I -- I'm counting on people I trust who told me this story so in terms of documentation, no, I can't prove it, but I can certainly say with 100% certainty that I was told this story by a source very close to Donald. Election night 2016, I think you tweeted out worst night of my life. One of them. One of the worst nights of my life. Then why go to the white house in April 2017? I had been on the outside of this family for a really long time, and after my cousin ivanka's wedding for reasons I still don't understand I was invited to, my aunt Maryanne and I started talking and we developed a relationship which we had never had before quite honestly and it mattered to me, you know. It was the first time I had felt part of the family since I was a kid and somehow it was very easy for me to put aside all of the things that had happened previous to that. When I got invited to her birthday party I felt that I should -- I should go. You go -- you see the president in the oval office and you tell him don't let them get you down. Did you mean that? I did actually. He -- that was four months in. He already seemed very strained by the pressures. I didn't mean I want you to keep doing what you're doing and get away with it but and also so much of what has happened since then hasn't yet happened and I thought his response was actually more enlightening than my statement and he said, they won't get me and so far looks like he's right. And if you're in the oval office today what, would you say to him? Resign. Blunt as she could be right we're going to have a lot more of this exclusive interview coming up in the next half hour and she'll talk about what she calls a malignantly dysfunctional family that all began, she says, with the father of president trump, Fred Sr. Robin.

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

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