A top Harvard constitutional law professor is calling for an impeachment investigation into Donald Trump for obstruction of justice, branding the president’s firing of FBI Director James Comey as an “obvious effort to interfere with a probe involving national security.”

To “wait for the results of the multiple investigations underway is to risk tying our nation’s fate to the whims of an authoritarian leader,” Professor Laurence Tribe wrote in an opinion piece in The Washington Post on Saturday. Tribe added that Trump’s conduct “strongly suggests that he poses a danger to our system of government.”

After Comey’s firing last week, Tribe joined the legal advisory board of Impeach Trump Now, a campaign pushing for an impeachment probe into the president.

“It is now up to Congress to save the Constitution by initiating impeachment proceedings,” Tribe said in a statement Friday when he joined the board. “Trump can’t say ‘You’re fired’ to the House of Representatives.”

Tribe wrote in the Post that “ample reasons existed to worry about this president, and to ponder the extraordinary remedy of impeachment even before the Comey firing.” He cited Trump’s apparent “brazen defiance” of the foreign emoluments clause of the Constitution, which bars a president from receiving foreign funds.

But Tribe is particularly troubled by Comey’s firing and its aftermath. After the ouster last week, Trump appeared to encourage Attorney General Jeff Sessions and his deputy Rod Rosenstein to provide cover for a decision Trump conceded he had already made and scrambled to work out some consistent “story” for Comey’s firing.

Tribe said Trump’s actions in firing Comey were “vastly more serious” than the “third-rate burglary” that Richard Nixon tried to cover up in Watergate. On Friday, journalist James Fallows wrote in the Atlantic that Trump’s scandal “looks worse than Watergate.”

“The crucial thing is that the prospect [of impeachment] now be taken seriously, that the machinery of removal be reactivated, and that the need to use it become the focus of political discourse going into 2018,” Tribe concluded.