WASHINGTON — When President Trump publicly demanded that the Justice Department open an investigation into the F.B.I.’s scrutiny of his campaign contacts with Russia, he inched further toward breaching an established constraint on executive power: The White House does not make decisions about individual law enforcement investigations.

“It’s an incredible historical moment,” said Rebecca Roiphe, a professor at New York Law School who helped write a coming scholarly article on the limits of presidential control over the Justice Department. Mr. Trump’s move, she said, “is the culmination of a lot of moments in which he has chipped away at prosecutorial independence, but this is a direct assault.”

Almost since he took office, Mr. Trump has battered the Justice Department’s independence indirectly — lamenting its failure to reopen a criminal investigation of Hillary Clinton that found no wrongdoing, and openly complaining that Attorney General Jeff Sessions recused himself from the Russia inquiry. But he had also acknowledged that as president, “I am not supposed to be involved with the Justice Department,” as he told a radio interviewer with frustration last fall.

As part of that pattern, he has also denied the account by James B. Comey, the F.B.I. director he abruptly fired, that the president privately urged him to drop an investigation into Michael T. Flynn, Mr. Trump’s first national security adviser.