WATCH: Presidential historian explains why Donald Trump is the ‘nightmare of founders coming true’

Impeachment proceedings against President Donald Trump could move "incredibly quickly" presidential historian Jon Meacham explained on MSNBC's "The Last Word" with Lawrence O'Donnell.

"We have never seen anything like the presidency of Donald Trump," O'Donnell noted. "But don't take that from me, take it from presidential historian John Meacham who has studied the history of the American presidency much more closely than I have."

The host played a clip of Meacham speaking on MSNBC's "Morning Joe" on Tuesday.

"This is an existential constitutional crisis because it's quite possible that the president of the United States right now is a witting or at least unwitting, partially-witting agent of a foreign power," Meacham said in the clip.

"What would your advice be to Democrats in the House of Representatives about how they should handle their powers next year in relation to what they're seeing in this existential constitutional crisis?" O'Donnell asked.

Meacham recounted a brief history of the three major impeachment efforts in congressional history.

"Here we are now with something that goes all the way back to a nightmare of the founders coming true," he explained.

"I think what I would recommend to House Democrats and Republicans would be to go read Federalist 22, which was Alexander Hamilton talking about the nefarious and pernicious possibility of foreign corruption on the new republican, lower-case 'r,' government," Meacham noted.

"There's a reason we have a clause in the Constitution that says you can't accept titles of nobility, you can't grant titles of nobility," he continued. "You can't accept gifts from foreign powers unless you have congressional approval."

"They were incredibly worried -- in the 1780s and '90s -- that the republican experiment would fall prey to exactly the kind of thing that Vladimir Putin, two-and-a-half centuries on, is doing," Meacham argued. "He's running a campaign to destabilize the government and the question is, and I think we'll have to see from director [Robert] Mueller, to what extent was candidate Trump -- and President Trump -- a knowing part of that conspiracy or to what extent did that conspiracy -- has it affected what he does?"

"Those are the questions, that depending on the answers to them, I think we could be in a dramatic place -- very quickly," he concluded.

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