Donald Trump's personal request that the FBI drop its investigation into disgraced national security adviser Michael Flynn, if true, could see the President's fledgling administration already entering "impeachment territory".
That's according to a former adviser to three presidents, two of whom - Richard Nixon and Bill Clinton - faced impeachment proceedings.
Mr Trump's reported comments to then-FBI chief James Comey represent the most concerning conduct yet by the 45th president, according to David Gergen. It comes after a tumultuous week in which Mr Trump admitted he fired Mr Comey at least partially because of the investigation into his team's links to Russia, and after the President passed apparently classified secrets to senior Russian officials.
Accoridng to notes Mr Comey made after a February meeting with Mr Trump, the President told him: “I hope you can see your way clear to letting this go, to letting Flynn go. He is a good guy. I hope you can let this go.” The memo was reported by US media, including the New York Times and CNN, on Tuesday evening.
The memo appeared to show Mr Trump “trying to impede the investigation” into alleged Russia links, according to Mr Gergen, who served as a top adviser to Presidents Clinton and Nixon, as well as to Ronald Reagan.
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He told CNN: “After watching the Clinton impeachment, I thought I would never see another one. But I think we’re in impeachment territory for the first time.
”I think that the obstruction of justice was the number one charge against Nixon that brought him down.
“I'm a lapsed lawyer, I can not tell you if it meets all of the legal definitions, but I can tell you from a lay point of view, it looks like [Mr Trump] was trying to impede the investigation, he was using his power to do that, and when James Comey didn't go along with him, he wasn't his boy, he fired him, which I think is also relevant to the question of what he was trying to do.
“So, from my point of view, this is of enormous consequence for his presidency.”
Meanwhile, David Axelrod, Barack Obama's campaign manager and former adviser, tweeted: “I've been resistant to impeachment talk until now, but if Comey memo is true - and Comey is very credible - we are into a whole new deal here.
”If @POTUS told Comey 2 “close down” the Flynn investigation, as Comey memo alleges, it may well lead to the closing down of his presidency.“
“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,” the White House said.
“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.”
Mr Comey's memo, an apparent effort to create a paper trail of his contacts with the White House, would be the clearest evidence to date that the President has tried to influence the Russia investigation.
Rep Jason Chaffetz, Republican chairman of the House oversight committee, sent a letter to the FBI on Tuesday requesting that it turn over all documents and recordings that detail communications between Mr Comey and Mr Trump. He said he would give the FBI a week and then "if we need a subpoena, we'll do it."
The panel's top Democrat, Elijah Cummings of Maryland, a constant Trump critic, called the allegation of White House pressure on Mr Comey "explosive" and said "it appears like a textbook case of criminal obstruction of justice."
John McCain, chairman of the Senate Armed Services Committee, said late Tuesday that the developments had reached "Watergate size and scale."
Mitch McConnell, the Republican leader of the Senate, said simply, "It would be helpful to have less drama emanating from the White House."
The White House vigorously denied it all. "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 statement said.
Mr Trump fired General Flynn on 13 February on grounds that he had misled Vice President Mike Pence and other officials about his contacts with Russians.
Additional reporting by agencies