Should President Trump have recused himself? Meaning, should he have refrained from trying to fire Special Counsel Mueller because he had a conflict of interest? The abiding assumption throughout the hearing of Mr. Mueller today in the Judiciary Committee seems to be that he should have. He should have stayed his hand, even if he knew he was innocent of having colluded with the Russians.

Our own view is that such pussyfooting is unconstitutional. That is because the presidency is unique, different than all other offices in our constitutional system. The president is the only officer whom the Constitution commands to take care that the laws be faithfully executed. The job is solely his. So he can’t recuse himself even if he has a conflict of interest. He just has to step up.

In law, the name for the principle is the “rule of necessity.” We first wrote about it in these columns a year ago. We quoted Merriam-Webster’s law dictionary as saying the “rule of necessity” is a principle “permitting or requiring a judge or other official to adjudicate a case despite bias or personal interest when disqualification would result in the lack of any competent tribunal.”

“Mark the way that is phrased,” our editorial said. “It’s not just that there are circumstances where the rule of necessity ‘permits’ a person to act despite a conflict of interest. There are times when the rule ‘requires’ him to act, even in the face of a conflict of interest. It requires him to act when disqualification — recusal, it’s sometimes called — would result in the ‘lack of any competent authority.’”

So to our editorial ear, the Judiciary Committee Democrats have it backwards. They keep nursing the notion that it would be obstruction if Mr. Trump tried to fire Mr. Mueller because Mr. Mueller was investigating the president. On the contrary, the rule of necessity suggests that the obstruction would come if Mr. Trump shrank from his duty and refused to fire Mr. Mueller, just because he was afraid of the controversy.

He swore, after all, an oath before God to preserve, protect, and defend the Constitution.

What about all the other officers who, from time to time, recuse themselves? Even a Supreme Court justice, after all, recuses himself or herself once in a while. They’re different. The Supreme Court can still function with eight justices. Congress, which created the associate justices in the first place, could even establish a bench of alternate justices to hang around until needed.

When Attorney General Sessions recused himself, his deputy, Rod Rosenstein, stepped up. That can’t be done, though, in respect of the president. In the case of the president, there is no one else who is ordained — as the president is in Article II of the Constitution — to take care that the laws be faithfully executed. Nor could anyone else even temporarily step in as commander-in-chief.

We have no illusions about this. The Judiciary Democrats are not going to stay their hand over a legal principle like the rule of necessity. The impeachment proceeding has already begun. That is the meaning of Chairman Nadler’s opening remarks, in which he noted that the Constitution requires a system other than that of criminal justice to formally accuse a sitting president of wrongdoing.

“That process begins with the work of this committee,” he said. And we’re off to the races. How far it will get is beyond our ken. We predict, though, that before this is over, one court or another — or the Senate — is going to wake up to the rule of necessity and confront the president’s constitutional authority to shut down an investigation he deems unnecessary, even if it protects himself.