“We’ve been advised that the president certainly has the power to make that decision,” she added.
But there is scant precedent supporting the notion that Mr. Trump has lawful authority to bypass the acting attorney general and directly fire Mr. Mueller, legal scholars said.
Martin S. Lederman, a Georgetown University law professor who served in the Office of Legal Counsel during the Obama administration, pointed to a series of Supreme Court cases in which justices across generations have said that the power to remove a government employee resides with the official — or a successor — who appointed the person, unless Congress has enacted a statute making an exception.
Among them, in a seminal 1839 case, the Supreme Court said the heads of departments appointed inferior officials and had the power to remove them, adding “the president has certainly no power to remove.” As recently as in a 2010 case, Chief Justice John G. Roberts Jr., observed that if Congress had vested in a department head the power to appoint an inferior officer, “it is ordinarily the department head, rather than the president, who enjoys the power of removal.”
Against that backdrop, Mr. Lederman argued: “Surely if Richard Nixon could have fired Archie Cox himself, he would have done so to avoid the Saturday Night Massacre. He didn’t because he didn’t have the authority to do so.”
Still, Jack L. Goldsmith, a Harvard Law School professor and former head of the Office of Legal Counsel in the Bush administration, said the previous Supreme Court statements did not arise in a context exactly like a situation in which Mr. Trump said he had the constitutional power to fire Mr. Mueller directly and then tried to do it. Mr. Goldsmith said it was unclear what would happen then.
“I agree that the proper and most legally sound route would be to order Rosenstein to dismiss Mueller and then fire him if he doesn’t comply,” Mr. Goldsmith said. “That is clearly the right way under the law. That is the right way under the precedents. And that would be the legally safer way to do it. But there is an argument that he could do so directly and that question has never been tested in a context exactly like this.”
Further complicating matters is the question of how much the Justice Department’s regulation for special counsels is protecting Mr. Mueller.
People who do not want the Trump administration to fire Mr. Mueller have taken comfort from the fact that when Mr. Rosenstein appointed Mr. Mueller, he stated that a portion of those regulations — including a provision that says only the attorney general can fire a special counsel and only for misconduct — would be “applicable” to Mr. Mueller.
In a statement criticizing Ms. Sanders’s statement on Tuesday, Matt House, a spokesman for the Senate minority leader, Senator Chuck Schumer, Democrat of New York, said, “The D.O.J. regulations could not be more clear.”
In the 1974 case over the Watergate tapes, the Supreme Court cited a similar regulation protecting Mr. Cox’s successor, Leon Jaworski, from being fired without good reason and said that unless it was rescinded, “the executive branch is bound by it.”
Still, one novel question is whether a president must obey a Justice Department regulation.
Another is whether the regulation is actually protecting Mr. Mueller from arbitrary firing by anyone other than Mr. Rosenstein, specialists said. The last question is raised by the fact that Mr. Rosenstein did not technically appoint Mr. Mueller under that regulation.
Rather, he invoked statutes in which Congress authorized the attorney general to appoint special lawyers for specific investigations, but those statutes include no special protections.
That has left it murky whether Mr. Rosenstein’s statement that the regulation applies to Mr. Mueller is legally binding or was more like a personal promise about the approach he would follow as Mr. Mueller’s supervisor. Still, Mr. Lederman said the distinction might make little difference in practice, since any successor to Mr. Rosenstein who was determined to fire Mr. Mueller could also just rescind Mr. Mueller’s appointment letter or the regulation itself.
“You’re already way out there if you’re willing to fire Mueller, so if you have to get rid of the regulation or the appointment letter to do so, that person, whoever that would be, would probably be happy to do so,” he said.