(CNN) If President Trump truly did ask James Comey to end the investigation into national security adviser Michael Flynn -- as the former FBI director is said to have claimed in a memo -- then the United States has reached a rare tipping point, David Gergen said Tuesday night.

"After watching the Clinton impeachment I thought I'd never see another one, but I think we're in impeachment territory now for the first time," he told Anderson Cooper during a live CNN appearance.

"I think we're in impeachment territory" @david_gergen reacts to news of James Comey memo https://t.co/yCgzdgP2f5 https://t.co/LXsxOXfuJ3 — Anderson Cooper 360° (@AC360) May 17, 2017

Gergen, who advised two US Presidents who faced impeachment processes -- Richard Nixon and Bill Clinton -- says it appears Donald Trump was intentionally trying to influence the direction of the FBI's probe into Flynn.

"It looks like he was trying to impede the investigation," said Gergen. "He was using his power to do that, and when James Comey didn't go along with him, when he wasn't his boy, he fired him."

Comey claimed in the memo that Trump said " I hope you can let this go ," referring to the Flynn probe, according to CNN sources who described the document.

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