Robert Mueller made a surprising assertion last month about the limits of his power. In his report on Russia’s interference in the 2016 election and President Trump’s potential obstruction of the investigation, the special counsel explained that Justice Department policy effectively prevented him from charging Trump with a crime while in office. But in his surprise press conference in May, he went even further. “[The report] explains that under long-standing department policy, a president cannot be charged with a federal crime while he is in office,” he said. “That is unconstitutional. Even if the charge is kept under seal and hidden from public view—that too is prohibited.”

This remains an open legal question, despite Mueller’s unequivocal assertion. The Constitution itself is silent on the matter, and no court has ever ruled otherwise because no sitting president has ever been indicted. Mueller’s nod to “long-standing department policy” likely was a reference to the so-called Dixon memo, a 1973 Office of Legal Counsel opinion in which Assistant Attorney General Robert Dixon concluded that there were multiple practical and constitutional hurdles that made it effectively impossible. “The spectacle of an indicted president still trying to serve as chief president boggles the imagination,” Dixon wrote.

That memo’s primary purpose, however, was not to conclusively decide whether a president could be indicted while in office. While it’s commonly assumed that the memo came about during the Watergate scandal, it instead sprang from the Justice Department’s efforts to prosecute Vice President Spiro Agnew in a tax-evasion case. Agnew argued that he was only subject to impeachment by Congress, and Attorney General Elliot Richardson asked Dixon to write an opinion on the question.

To understand the Dixon memo’s unusual origins and its continuing impact, I spoke with J. T. Smith, an attorney who worked as Richardson’s executive assistant during the Watergate scandal. Smith was present at the creation, so to speak, of the Justice Department’s policy on indicting a sitting president. He told me that if Richardson “had the benefit or detriment we have of the behavior of this particular White House, he almost certainly would say it’s high time this whole matter get revisited.”

Did you happen to see Mueller’s press conference the other day, where he said outright that it would be unconstitutional to indict a sitting president?

