Giuliani biographer on CNN: Hero of 9/11 is now 'unhinged' and has 'severely damaged his legacy'
Giuliani biographer Andrew Kirtzman appears on CNN/Screenshot

Rudy Giuliani's stunning admissions that Donald Trump repaid his lawyer for hush-money given to a porn star he had sex with for the express purpose of preserving his reputation in the waning days of the 2016 presidential campaign has put the president's new lawyer in the spotlight.

Is Giuliani, who is 73 years old, losing a step? His biographer thinks so.

Andrew Kirtzman is the author of Rudy Giuliani: Emperor of the City, which came out in the post-9/11 era when the New Yorker was "America's Mayor." He went on CNN on Thursday to talk about the man he profiled nearly two decades ago, fresh out of the ash of the World Trade Centers.

Kirtzman said that Giuliani was once a great lawyer—something others disagree with—and as a prosecutor, widely respected and feared.

"Giuliani was a brilliant tactician, both as prosecutor as mayor," he said. "On 9/11, his bombast almost helped, because he was courageous, he was fearless... he was always bombastic, but he was always careful to achieve his goals. There was always a strategy to it. And now we're seeing, in recent years not just this morning as somewhat more unhinged."

Part of this issue is that Giuliani is just more extreme.

"Over the years he has turned far more extreme," he said. "Now, talking about the FBI as Stormtroopers."

Because of that, the Giuliani Kirtzman wrote about is gone.

"The hero of 9/11 is somewhat being forgotten—he has severely damaged his legacy," Kirtzman said.

Watch below.