On Sunday, Trump lashed out against Schiff, who heads up the House Intelligence Committee, accusing the Californian Congressman of “Fraud & Treason.”

The attack follows a speech Schiff made in front of the House Intelligence Committee on Thursday, in which he parodied the president during his now-infamous phone call with the Ukraine President Volodymyr Zelensky on July 25.

During that phone call, Trump requested “a favor” involving the launch an investigation into the business dealings of Democratic presidential candidate Joe Biden’s son, Hunter—despite no actual evidence of wrongdoing by either the 2020 hopeful or his son.

“His lies were made in perhaps the most blatant and sinister manner ever seen in the great Chamber,” Trump said of Schiff on Twitter.

“He wrote down and read terrible things, then said it was from the mouth of the President of the United States. I want Schiff questioned at the highest level for Fraud & Treason…..”

David Burbach, an associate professor in international relations at the US Naval War College in Rhode Island, retweeted Trump, highlighting the threatening use of language used by the President.

“You can not brush it off as ‘he just talks that way,’” said Burbach. “This is a real head of state with great legal authority in charge of US Marshalls and FBI agents and an awesome military machine. This. Is. For. Real.”

Burbach responded to a second tweet that Trump posted less than three hours later, quoting a contributor on Fox News, Pastor Robert Jeffress, invoking the Civil War. Jeffress said that a successful impeachment would create the conditions for a “Civil War like fracture.”

“A quote vs his own words, but this is still a terribly irresponsible thing for the President to say,” said Burbach, whose specialisms include civil-military relations in addition to international relations, national security and the politics of U.S. foreign policy.

Burbach told Newsweek Trump’s tweets are “inflammatory” and display “dangerous rhetoric for a President,” adding these words could inspire violence against the whistleblower. Trump supporters have already issued a $50,000 “bounty” for “any information” about their identity, lawyers have revealed.

“This is chilling to any federal employee—and likely meant to be chilling,” said Burbach, who made it clear he was speaking personally and not on behalf of the Naval War College.

“Trump hinting, even obliquely, that violence would result if Congress lawfully impeaches and convicts him is grossly inappropriate for the head of state—but it is consistent with Rudy Guiliani tweeting earlier that “domestic tranquility” will not be maintained if the Democrats move forward with impeachment.”

The comments Trump made on Sunday were just the latest in a series of tweets directed at Schiff over the last few days, who he has called “desperate” and “a sick man” while calling for his immediate resignation.

On Thursday, at an event at Andrews Air Force Base, he accused Schiff of “making up stories.” That was just hours after he suggested the source who tipped off the whistleblower responsible for breaking the Ukrainian phone call story should face “big consequences.”

Others have also condemned Trump’s tweets against Schiff, including the Republican Congressman for Illinois’ 16th District, Adam Kinzinger.

Laurence Tribe, a legal scholar and Carl M. Loeb University Professor at the Harvard Law School, called it “a grave abuse of his bully pulpit,” describing Trump as “a menace to our democracy.”

This article has been updated to include Prof. David Burbach’s comments.