AMNA NAWAZ: It wasn't that long ago, in 2012, that Mitt Romney was the Republican Party's presidential nominee.

But, today, the GOP he once led is dominated by Donald Trump.

Now a senator from Utah, Romney is attempting to make sense of the changes within the party and his own actions over his decades-long political career.

For numerous hours over 14 months, he shared his thoughts with journalist McKay Coppins.

Lisa Desjardins picks up the story from there.

LISA DESJARDINS: That's right, Amna.

And the result of those intimate conversations is McKay Coppins new book, "Romney: A Reckoning."

And McKay joins me now.

McKay, lots of headlines from this, but Mitt Romney here is not just reckoning with his party, but also reckoning with himself in some ways.

And I wonder, can you pick a revealing moment that shows sort of this restless quality that comes through in the book, and how this affects a historic figure making big decisions throughout?

MCKAY COPPINS, Author, "Romney: A Reckoning": Yes, it's interesting.

One of the things that drew me to him as a subject was that, especially in the wake of January 6, he was really going through a process of trying to understand how his party had become the way it was and whether he played any role in the party's de-evolution.

Now, obviously, he's been a prominent critic of Donald Trump's, but he told me about a moment in the wake of the 2016 election when he had been very openly hostile to Donald Trump's campaign, but Trump actually invited him to talk about joining his administration as secretary of state.

And Romney went back and forth on whether to pursue that, and he did have a couple meetings with Trump.

And he told me: If I'm being honest with myself, I said that I had a lot of noble intentions in pursuing this, but there was also a part of me that just wanted the job, wanted the power, wanted to be in the middle of the action.

And I think what's fascinating about my conversations with him is that he was often going back through his career and his life and identifying those moments where he was rationalizing things in his self-interest.

And I think that's important, because I think that those kinds of rationalizations run through a lot of American politics today, and it's what's brought us to this moment that we're in right now.

LISA DESJARDINS: How does he look at his role with Trump and whether he's complicit with where Trump ended up?

MCKAY COPPINS: There's one moment in the book, and some people might remember this.

In 2012 when Mitt Romney was running for the Republican presidential nomination, he accepted Donald Trump's endorsement.

At the time, Mitt Romney rationalized to himself that Trump was a celebrity.

He was a kind of buffoonish figure, and both parties have celebrities like this.

But in the years since, obviously, that moment has looked worse and worse to Romney.

And he told me: If there's anything that I did in that campaign to give him credibility and help him four years later become president, obviously, I regret it.

And I think that he does have a lot of regret about not just how he indulged Trump and kind of the MAGA wing of the party, but, more broadly, how he indulged some of the more toxic elements of his party when he was trying to pursue the presidency.

A lot of the story of this book is those extremist forces in the party that people like Mitt Romney thought they could sort of keep at bay and that, over the years, especially recently, kind of took over the party, and they realized that it wasn't possible to just sort of flirt with them and then keep them at arm's length.

LISA DESJARDINS: This book has made a lot of news.

It also has garnered a lot of reaction.

Some of my sources on Capitol Hill, including some Romney allies, actually felt that there were some hypocritical kind of ideas in here for Mr. Romney.

How do you reckon these two Mitt Romneys?

Which one is real, the one who is the most venting sharply to you behind closed doors or the one who is not saying those same things in public yet?

MCKAY COPPINS: Well, a lot of the venting that he did for -- with me and that I recall in the book was stuff that he had said over the course of years.

So, he gave me his private journals that recounted some of the stories of his interactions with people like Mitch McConnell, Ted Cruz, Josh Hawley.

And it's funny.

He later told me that he hadn't reread those journals before he gave them to me.

So I put them in there to demonstrate his growing alarm with the leadership of his party over years.

But, I mean, look the reality is he has been very critical of the leadership of his party.

He's enormously disappointed in some of his colleagues and how they, behind closed doors, will say that Donald Trump is a menace, that he's a buffoon, and then, in public, will perform their loyalty to him.

And I think he deserves some credit for saying it out loud, the things that most of his Republican colleagues would never say out loud.

LISA DESJARDINS: What is his relationship with the Republican Party, you think, going forward?

MCKAY COPPINS: I think he's more isolated than ever.

I mean, he has made very clear that he doesn't have a home in this party anymore.

He, by the end time of -- by the end of our conversations, had been openly talking to me about leaving the party, starting a third party, perhaps.

As long as Donald Trump is the dominant figure in the GOP, he is going to remain a vocal critic and opponent of significant elements of that party.

And I will be interested to see if he remains affiliated with it in the years ahead.

LISA DESJARDINS: His book is "Romney: A Reckoning."

McKay Coppins, thank you for all the work and for joining us.

MCKAY COPPINS: Thank you.