Presidential historian Jon Meacham said Tuesday that President Trump Donald John TrumpBiden on Trump's refusal to commit to peaceful transfer of power: 'What country are we in?' Romney: 'Unthinkable and unacceptable' to not commit to peaceful transition of power Two Louisville police officers shot amid Breonna Taylor grand jury protests 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 Mueller Robert (Bob) MuellerCNN's Toobin warns McCabe is in 'perilous condition' with emboldened Trump CNN anchor rips Trump over Stone while evoking Clinton-Lynch tarmac meeting The Hill's 12:30 Report: New Hampshire fallout MORE continues to investigate Russian election interference, including whether the Trump campaign colluded with Moscow. Trump associates Michael Flynn, George Papadopoulos, Paul 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 Sessions Jefferson (Jeff) Beauregard SessionsGOP set to release controversial Biden report Trump's policies on refugees are as simple as ABCs Ocasio-Cortez, Velázquez call for convention to decide Puerto Rico status 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.