Here, the evidence strongly suggests that the president acted corruptly. That starts with the demand for loyalty from Mr. Comey, the account of which the White House disputes. That demand can reasonably be understood to mean that Mr. Comey should protect Trump and follow his bidding, rather than honoring his oath to follow the evidence. It is also an implicit threat: Be loyal, or you will be fired.

When Mr. Comey did not seem to take the hint, Mr. Trump made his meaning crystal-clear on Feb. 14: Let the investigation go, and let Mr. Flynn go, too. The president denies this as well, of course, as he has denied so much else that has proven to be true. Who are we to believe: Mr. Comey, who would have no reason to accuse the president of obstruction of justice, and who has apparently preserved meticulous notes of his conversations? Or the president, who fact-checkers have demonstrated has told more lies in less time than any other modern occupant of the Oval Office?

While Mr. Trump might have been within his rights to fire Mr. Comey, this pattern of demands to protect himself and Mr. Flynn, followed by retaliation when the demands were not met, if proven, is a textbook case of wrongful conduct. Add to this the fact that Mr. Flynn was already offering testimony about the Russia connection in exchange for immunity from prosecution, and Mr. Trump’s clumsy attempt to dissemble the cause of the firing, and it is clear that a cover-up was afoot.

Finally, Mr. Trump topped things off with his tweeted threat to Mr. Comey; witness intimidation is both obstruction of justice in itself, and a free-standing statutory offense.

Taken together, this evidence is already more than sufficient to make out a prima facie case of obstruction of justice — and there are likely many more shoes to drop. Mr. Comey reportedly took notes on all of his encounters with the president. If what has emerged so far is any indication, this is unlikely to offer much comfort to Mr. Trump.

And there remains the core question of the president’s motives. Is he withholding his taxes because they show evidence of “a lot of money pouring in from Russia,” as his son once stated, or do they show no such thing, as his lawyers claim? Why is Mr. Trump so fervently protecting Mr. Flynn: out of loyalty to a friend, or because Mr. Trump fears what that friend would say if he received immunity?

We have previously called for Congress to set up an independent 9/11-style commission on the Russia and Flynn investigations, and for the Department of Justice to appoint a special prosecutor. This appointment is necessary because Congress can’t actually prosecute anyone who may have committed crimes, including obstruction of justice, in connection with the Trump-Russia matter. This week’s revelations about the president, the most powerful man in the country, emphasize the need for these independent structures to be erected and to encompass these new allegations.