Trump has obliterated all of those expectations.

Now comes another one shattered, with last week’s report in the New York Times that the president wanted to order the Justice Department to prosecute two of his political adversaries, his 2016 opponent Hillary Clinton and former FBI director James B. Comey.

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Trump reportedly was talked out of it by then-White House counsel Donald McGahn. But McGahn is now gone, and Trump appears to be still mulling the idea, even as he rages about a twin obsession: special counsel Robert S. Mueller III’s investigation.

As recently as Nov. 15, Trump groused on Twitter: “They won’t even look at all of the bad acts and crimes on the other side.”

The president has also fired Attorney General Jeff Sessions, whom he considered insufficiently loyal, and put in his place, at least temporarily, Matthew G. Whitaker, a vocal critic of the Mueller probe and presumably someone Trump views as more likely to look after his interests.

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All of this is a reminder that the premise that presidents will keep their fingers off the scales of justice is a relatively recent development in our history, dating only as far back as the outrage over Richard M. Nixon’s abuses that were uncovered during Watergate.

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Nixon was far from the first to see no barrier between the White House and the Justice Department. Theodore Roosevelt stage-managed antitrust prosecutions. Lyndon B. Johnson had the FBI spy on rival factions in his own party at the 1964 Democratic convention. And, most famously, John F. Kennedy appointed his brother as attorney general.

What grew up after Watergate was an informal policy, initiated by President Gerald Ford, under which presidents limit the number of White House officials who could communicate with the Justice Department about active investigations, and the circumstances under which they could do so.

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But presidents do not have to make those policies public and can change them at will.

That should be fixed, and a bipartisan task force put together by the Brennan Center for Justice at the New York University School of Law has come up with some sensible, restrained ways to do it.

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The proposals are included in a wide-ranging set of ideas for reform that the task force put out in early October.

Among them: a requirement that every administration publish its policies for contacts between the White House and law enforcement agencies, and that law enforcement agencies maintain a log when they hear from the White House regarding specific criminal or civil enforcement matters. Relevant congressional committees and the Justice Department’s inspector general would have the ability to inspect those logs.

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“If someone knows there will be a record of their contact, they will likely take care to ensure it is appropriate,” noted the task force, which was co-chaired by former New Jersey governor Christine Todd Whitman (R) and Preet Bharara, the former U.S. attorney for the Southern District of New York who was fired by Trump in March 2017.

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That no such record currently exists was one of the concerns that prompted Comey to start writing memos to himself about his early conversations with Trump. They contained some explosive allegations, including that the newly inaugurated president demanded loyalty from his soon-to-be-fired FBI director, and that Trump argued the agency should drop its investigation of contacts with Russia by his first national security adviser, Michael Flynn.

The presidency is an institution invested with enormous power and enormous leeway for how to use that power. Trump’s two years in office have revealed how few legal and political tools there are for curbing a chief executive who does not feel constrained by norms — or, for that matter, by shame, ethical standards or fear of public backlash.

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Nowhere is the potential for overreach more dangerous, however, than in the application of our laws. Using that power to reward friends and punish adversaries marks the dividing line where a president becomes a tyrant — and it is one that Trump seems determined to cross.