The case was assigned to Judge Trevor N. McFadden, a Trump appointee.

The Justice Department last week disclosed an Office of Legal Counsel memo arguing that Mr. Whitaker’s temporary appointment was lawful under the Vacancies Reform Act of 1998. It also cited an 1898 Supreme Court opinion and numerous historical examples in support of the proposition that an office whose holder must normally be Senate-confirmed can be temporarily filled by an acting official who has not gone through the confirmation process.

“There are over 160 instances in American history in which non-Senate-confirmed persons performed, on a temporary basis, the duties of a Senate-confirmed position,” Kerri Kupec, a Justice Department spokeswoman, said in a statement on Monday. “To suggest otherwise is to ignore centuries of practice and precedent.”

The Justice Department memo pointed to various examples of such a temporary appointment dating back to the earliest days of American history, although it acknowledged that there was very little precedent for an acting attorney general who did not undergo confirmation. There was such an acting attorney general in 1866 for several days, but that was before Congress had created a Justice Department for that official to supervise.

In an interview, Mr. Blumenthal said that most of the historical examples the administration had cited involved short-term appointments demanding immediate attention whose constitutional legitimacy was never tested in court. He also argued that the attorney general’s special powers set that office apart.

“In modern times, this kind of appointment is unprecedented, certainly for an office of this importance,” he said.

In the House, where Democrats will assume control in January, they may soon have more leverage. Representative Jerrold Nadler of New York, the incoming chairman of the Judiciary Committee, has pledged to make Mr. Whitaker his first hearing witness and to subpoena him if necessary. He has warned that there would be consequences for firing Mr. Mueller.

And in a series of letters, Mr. Nadler and other soon-to-be Democratic chairman have put the Justice Department and the White House on notice that they intend to investigate the circumstances of Mr. Sessions’s removal and replacement, and will request access to communications related to both.

But the warnings highlight the dilemma for lawmakers worried that Mr. Trump or his associates might act to curtail the inquiry. Even though Democrats can use the House majority to try to check those impulses, they have no real say over presidential appointments — and in any case, it will be several more weeks until they actually gain access to the levers of power.