Except during Senate recesses, the Appointments Clause of Article II requires that the president nominate and the Senate confirm all “principal” federal officers. Clearly, the attorney general of the United States is a principal officer, and so the president could not permanently fill that post without the Senate’s advice and consent. The argument against Mr. Whitaker’s appointment rises and falls on the assumption that someone who temporarily exercises the duties of a principal officer must be a principal officer — or at the very least, an “inferior officer” like the deputy attorney general or solicitor general, who have already been confirmed by the Senate to those posts.

But in an 1898 decision, United States v. Eaton, the Supreme Court rejected the argument that only a principal officer confirmed by the Senate can temporarily fill the shoes of another principal officer. So long as an inferior officer is exercising the duties of the principal officer “for a limited time, and under special and temporary conditions,” the court said, he “is not thereby transformed into the superior and permanent official.” The Supreme Court in Eaton did not go on to define what “a limited time” or “special and temporary conditions” entails, but it made clear that those are the key constitutional considerations.

In the Federal Vacancies Reform Act of 1998, Congress elaborated on those missing pieces. When a senior executive branch officer “dies, resigns, or is otherwise unable to perform the functions and duties of the office,” the statute authorizes the president to choose either that official’s “first assistant” (in this case, Deputy Attorney General Rosenstein); any other currently serving government officer who was confirmed by the Senate; or any senior official, like Mr. Whitaker, who served in the same department as the vacant office for at least 90 of the previous 365 days “to perform the functions and duties of the vacant office temporarily in an acting capacity.”

A separate provision of the statute limits that designation to 210 days, unless the president nominates a permanent successor in the interim. Other laws distinguish between those duties Mr. Whitaker may carry out as acting attorney general and those limited to an attorney general confirmed by the Senate. For instance, the acting attorney general is expressly allowed to sign an application for a warrant under the Foreign Intelligence Surveillance Act but is expressly excluded from the line of succession to the presidency.