As investigations deepen into whether President Donald Trump was involved with Russian interference in the 2016 election, renowned presidential biographer Jon Meacham is arguing the president might be committing treason right now.

Meacham, during an appearance Tuesday on MSNBC’s “Morning Joe,” likened Trump to former President Richard Nixon, telling host Joe Scarborough that Nixon’s claim he was “not a crook ... would have fit on Twitter.” Meacham’s appearance on the show was flagged by Contemptor, a news media site.

Comparisons of Trump and Nixon have become a common theme among Trump critics, who have pointed out parallels in behavior and rhetoric. The heart of questions about Trump’s culpability, Meacham explained, is whether he knew about Russia’s involvement in the election and how he responded.

“If, in fact ... Donald 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 said.

“So, this isn’t just a who-wins-the-week, who-loses-the-week conversation. 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.”

Treason aside, the historian noted that the economy may also have a role in Trump’s presidential future. When Nixon’s presidency unraveled amid the Watergate scandal, he pointed out, the economy was turning sour.

“One barometer of what’s going to happen to President Trump is going to be in the corner of everyone’s television sets today, which is what the markets are doing,” Meacham said, noting that Trump has so far kept the backing of “401(k) Trump supporters” satisfied by the economy, even if the president’s behavior troubles them.

“Because their numbers were up, they were willing to give him a pass on the rest of it,” Meacham said of these Trump fans. “When that begins to crack, I think that goes away.”