(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.
Gergen: 'We're in impeachment territory now'
"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.
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.
Multiple White House officials refuted the claim on Tuesday, including one who said a "conversation of that nature" did not happen.
"While the President has repeatedly expressed his view that General Flynn is a decent man who served and protected our country, the President has never asked Mr. Comey or anyone else to end any investigation, including any investigation involving General Flynn," a White House official said in a statement. "The President has the utmost respect for our law enforcement agencies, and all investigations. This is not a truthful or accurate portrayal of the conversation between the President and Mr. Comey."
Citing a slew of recent controversies -- including Comey's messy dismissal, as well as recent reports of Trump sharing classified information with Russia -- Gergen said that things appear to be unraveling at 1600 Pennsylvania Avenue.
"What we see is a presidency that's starting to come apart," he said.