On Tuesday, leading presidential historian and frequent Trump critic Douglas Brinkley joined a chorus of voices denouncing the president's dismissal of FBI director James Comey.

"It's a very troubling abuse of power," Brinkley told Anderson Cooper on CNN. "Donald Trump just behaved like a tyrant. We see that in other countries in the world. We don't expect it here." 

Brinkley also addressed CNN senior legal analyst Jeffrey Toobin's earlier assessment of the parallels between the presidencies of Donald Trump and Richard Nixon.

"What was important about [the Saturday Night Massacre in 1973, when Nixon filed Watergate special prosecutor Archibald Cox], was that we had resignations, two real profiles of courage: Attorney General Elliot Richardson and Deputy Attorney General William Ruckelshaus [who] said, This is not okay, we're putting our country above party, and they quit," echoed Brinkley.

"And after that Saturday Night Massacre, eventually just like Donald Trump didn't want to talk a lot now, Nixon didn't want to talk a lot," he added. "When Nixon did come out of the woodwork, he famously said, 'I'm not a crook.'"

The Saturday Night Massacre marked an important turning point in public opinion. For the first time, a majority of the population supported the impeachment of President Nixon.

"The fact that Donald Trump is trying to exonerate himself is deeply troubling, I think, to everybody," said Brinkley. "The only historical precedent to it, Anderson, really is that Saturday Night Massacre."

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