Presidential historian: Trump's actions may be 'definition of treason' if he knew about Russian interference efforts

Presidential historian Jon Meacham said Tuesday that President TrumpDonald John TrumpTrump mocks wind power: 'When the wind doesn't blow, just turn off the television' Pentagon investigator probing whether acting chief boosted former employer Boeing Trump blasts McCain, bemoans not getting 'thank you' for funeral MORE's actions may represent the "definition of treason" if he knew of Russia's attempts to interfere in the 2016 election to help him win. 

“If, in fact, and it’s a subjunctive, Trump knew about the Russian efforts on his behalf, then there is a live question about whether he has been giving aid and comfort to the enemy, which is the definition of treason in the Constitution,” Meacham, who has written biographies on past presidents Thomas Jefferson and George H.W. Bush, said on MSNBC's "Morning Joe."

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“So, this isn’t just a who-wins-the-week, who-loses-the-week conversation," he continued. "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 partially witting agent of a foreign power, and I say that with great care, but that’s a possibility.

"And I think that we have to somewhat tap the brakes on our ordinary assessment of things and realize that this is possibly the biggest kind of question we could confront as a democracy."

Meacham's remarks came as he discussed the Watergate investigation of President Nixon. He noted that Nixon's downfall coincided with investigations and an economic downturn.

"What begins to happen in 1974, is you have the investigations, but you also have the economy souring," he said. "And that’s something to watch. One barometer about what’s going to happen to President Trump is ... what the markets are doing."

Special counsel Robert MuellerRobert Swan MuellerSasse: US should applaud choice of Mueller to lead Russia probe MORE continues to investigate Russian election interference, including whether the Trump campaign colluded with Moscow. Trump associates Michael Flynn, George PapadopoulosPaul Manafort, Richard Gates, Michael Cohen and more than 20 Russians have been charged in connection to the probe.

Trump has repeatedly derided the investigation as a "witch hunt" and on Sunday said that former Attorney General Jeff SessionsJefferson (Jeff) Beauregard SessionsRosenstein still working at DOJ despite plans to leave in mid-March Juan Williams: Don't rule out impeaching Trump O'Rourke on impeachment: 2020 vote may be best way to 'resolve' Trump MORE "should be ashamed of himself" for allowing it to proceed. 

His attorney Rudy Giuliani said Tuesday that Trump's legal team "might agree" to respond to more written questions from Mueller. The statement came after Giuliani said Sunday that Mueller would interview Trump "over my dead body."

— This story was updated on Feb. 13, 2019 at 11:04 p.m. to clarify a quote by Meacham.