“Trump’s saying he wanted a marionette whose strings he could pull to shutter the investigation,” said Adam Goldberg, who was a special associate White House counsel to President Bill Clinton during various investigations. “I’m sure the cronies who broke the law and are now convicted felons agree.”

Senator Chuck Schumer of New York, the Democratic minority leader, went to the floor of the Senate on Tuesday to cite the latest tweet as proof that Mr. Trump thinks he is above the law.

“The president’s tweet regarding Attorney General Sessions this morning is part of a pattern, where the president admits out loud and shamelessly that he was trying to take steps to end the Russia probe,” Mr. Schumer said. “This latest, stunning admission,” he added, “is just more evidence that the president may have something to hide. If he did nothing wrong, President Trump should welcome a thorough investigation to exonerate him.”

Still, even some scholars who are not on Mr. Trump’s payroll argue that the legal situation is not that clear-cut, even if the intent of Tuesday’s tweet was. The president, in this view, does not need to browbeat Mr. Sessions about ending the investigation when he could simply order it scuttled on his own.

“Yes, that is an explicit statement that Trump wanted an attorney general who would shut down the Russia investigation and is mad at Sessions for recusing himself and not shutting it down,” said Jack L. Goldsmith, a Harvard Law School professor and top Justice Department official under President George W. Bush. “That sounds bad. It sounds like Trump wants to interfere in the investigation.”

But Mr. Goldsmith, who has been a critic of Mr. Trump’s, nonetheless said that the president had “the constitutional authority to supervise criminal investigations” and could fire Mr. Sessions for recusing himself. An honest and innocent president could still conclude that an investigation like this was so without merit and so damaging to foreign policy that it should be ended, not out of self-interest.

“Everyone is assuming that when Trump makes these now commonplace threats, he is acting with corrupt intent, and maybe he is,” Mr. Goldsmith added. “But that has to be proved, probably over a very high bar. I am not exactly sure at this point what the corrupt intent is. The fact that Trump is so open and brazen about all this actually might make it harder for Mueller to show corrupt intent, though we don’t know what Mueller knows.”