GEOFF BENNETT: Senate Republican Leader Mitch McConnell, the longest-serving Senate leader in history, is set to step down from that position next month.

Over nearly two decades in the role, he has become one of the most consequential and powerful senators in U.S. history, reshaping the federal judiciary, now navigating a party transformed by Donald Trump.

The Associated Press' Michael Tackett has written the first definitive biography of Mitch McConnell due out on Tuesday.

It's called "The Price of Power: How Mitch McConnell Mastered the Senate, Changed America, and Lost His Party."

Michael Tackett joins us now.

Thanks for being here.

MICHAEL TACKETT, Author, "The Price of Power: How Mitch McConnell Mastered the Senate, Changed America, and Lost His Party": Oh, great to be here, Geoff.

GEOFF BENNETT: So this book has already garnered a lot of attention, in large part because you quote Mitch McConnell as he is sharing his private views with you about how he views Donald Trump.

He called him a sleazeball, a narcissist, says he's stupid, as well as being ill-tempered, not very smart, irascible, nasty, just about every quality you would not want somebody to have.

And yet Mitch McConnell has endorsed Donald Trump.

Why?

MICHAEL TACKETT: there'S a real contradiction there, obviously, and it's hard for people to sort that out.

What it starts with is, he wants to be in control of the Senate.

He wants to stay as the Senate leader.

He can't stay as a Senate leader if he doesn't endorse the Republican nominee.

So that's why he did it.

GEOFF BENNETT: He also says the MAGA movement is completely wrong, that Ronald Reagan wouldn't recognize the party today.

How does he explain Donald Trump's hold on the current Republican Party?

MICHAEL TACKETT: He thinks that there are a group of people who -- in the country who feel like life has not been fair, they have not been able to partake of the greatness of the country.

They haven't had the economic opportunity that they want to have, and therefore Donald Trump was able to capture them and keep them in their fold.

And people like him or traditional Republicans really find it hard to navigate.

GEOFF BENNETT: He's also said that he's proudest of shaping the Supreme Court, that his legacy will be defined by that.

And he considers his denial of Merrick Garland his proudest achievement, based on your reporting.

Democrats see that as the height of partisan obstructionism that really damaged the legitimacy of the court.

MICHAEL TACKETT: That's the debate.

And the thing is, is that what it really was a real blunt-force use of his power.

There really wasn't a clear precedent for that.

And some would argue there was no precedent for that.

But he could do it simply because he could, because, in the Senate, he only needed to have a majority of the votes to do it.

And he did it, and he rammed it through, and he was criticized for that.

And that's a big part of his legacy.

He looks at it as this was the greatest thing I ever did.

Others think he undermined the institution of the court and undermined the faith of the court in the process.

GEOFF BENNETT: He spoke with Justice Alito as you were reporting out this book.

What did he tell you?

MICHAEL TACKETT: Well, he agrees that McConnell really steered the court in the right direction from his point of view.

And it was interesting to me that Geoff, that he actually refers to it as the McConnell court, not the Trump court, the McConnell court.

GEOFF BENNETT: Let's talk about January 6, because after the rioters had been cleared from the Capitol and lawmakers returned from their secure location, McConnell addressed his staff.

And as you write, he started to sob softly.

He told his staff: "You are my family, and I hate the fact that you had to go through this."

He eventually voted to equip Donald Trump in the second impeachment that was focused on Trump's involvement in January 6.

But he didn't necessarily get to that decision easily, based on your reporting.

Walk us through that.

MICHAEL TACKETT: There's no question about that.

I think initially he thought -- first of all, he thought there was no question that he committed an impeachable offense.

He still thinks that.

The issue for him was, could you convict somebody if they were no longer in office?

And that was the rationale that he chose to use, that nobody had ever been impeached when they were out of office.

It's a really important part of understanding McConnell.

Like him or not like him, he is a rational actor.

He looks at the cost-benefit analysis.

If he doesn't see it, he doesn't do it.

And in this particular case, his calculation was, Donald Trump is going to fade from the scene.

He will be gone.

And I need the energy of the Trump base in the midterm elections.

And as I say in the book, it was the worst political calculation of his career.

GEOFF BENNETT: Does he view it that way?

MICHAEL TACKETT: I think he looks at it in hindsight and realizes that Donald Trump is still on the scene.

So he still thinks that there's no way he could have gotten 67 votes to convict.

And he thought there was no point in going through the exercise if it wasn't going to be successful.

GEOFF BENNETT: Where does he think the Republican Party goes next?

I mean, is Trumpism cemented within the GOP, in his view?

MICHAEL TACKETT: I think we will know the answer to that after Election Day.

If the election turns out to be favorable to the Republican Party, then they are clearly completely in the hold of Donald Trump.

If Republicans don't have such a good day, then there's a reckoning of a different kind.

GEOFF BENNETT: The title of the book is "The Price of Power."

What price has Mitch McConnell paid in the pursuit of power?

MICHAEL TACKETT: He said this more than once.

He said, look, I know all the Democrats hate me because of Garland.

I know half the Republicans hate me because of Trump.

And so he's also one of the least popular members of the U.S. Senate if you look at all the polling.

Even in his home state of Kentucky, his poll numbers are terrible.

And yet every time he runs for reelection, his margins grow.

So I think he's at peace with the fact that he did what he wanted to do.

But on the other hand, he knows he paid a heavy price in terms of sort of the isolation almost of being that unpopular.

GEOFF BENNETT: The book is "The Price of Power."

Its author is Michael Tackett.

Thanks again for speaking with me.

I appreciate it.

MICHAEL TACKETT: Thank you, Geoff.