President Donald Trump's repeated attempts to improperly influence federal judges in pending criminal cases and misuse the Justice Department to protect cronies accused of crimes has been audacious enough that it prompted the Federal Judges Association to schedule an emergency meeting of its executive committee on Wednesday morning and more than 2,000 former Justice Department officials from both parties to call for the resignation of Attorney General William Barr.

But his actions are not entirely without precedent: President Richard Nixon also improperly tried to influence federal judicial rulings in criminal cases, offering carrots for cooperation. Trump, though, has employed a different tack by swinging the cudgel of his Twitter account to intimidate judges.

Trump's latest attempt at public intimidation was aimed at U.S. District Judge Amy Berman Jackson, who is scheduled to sentence Trump associate Roger Stone on Thursday following his conviction by a jury on seven felony counts.

Last week Trump tweeted: "Is this the Judge that put Paul Manafort in SOLITARY CONFINEMENT, something that not even mobster Al Capone had to endure? How did she treat Crooked Hillary Clinton? Just asking!" He then followed up Tuesday with a series of tweets quoting a Fox News analyst suggesting that any other judge would acquiesce to calls for a new trial based on alleged statements made by the jury foreperson, but "I'm not so sure about Judge Jackson, I don't know."

This is only the most recent Trump attack — first as a candidate and now as the president — on federal judges.

During the 2016 campaign, Trump repeatedly attacked the impartiality of the judge presiding over the class action lawsuit against Trump University, U.S. District Judge Gonzalo Curiel, calling him a "hater" and a "Mexican," even though the judge was born in Indiana.

In 2017, Trump attacked U.S. District Judge James Robart, appointed by a Republican president, as a "so-called judge" who "put our country in peril" when he temporarily enjoined Trump's first executive order to ban travel from seven predominantly Muslim countries.

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In November 2018, when U.S. District Judge Jon S. Tigar in California temporarily halted Trump's effort to bar asylum to immigrants who had crossed the border between ports of entry, Trump tweeted that the ruling was a "disgrace" made by an "Obama judge."

In a rare public statement, Chief Justice John Roberts responded that "we do not have Obama judges or Trump judges," adding, "The independent judiciary is something we should all be thankful for."

Nixon's approach to influencing the judiciary was more subtle and less public than Trump's but just as pernicious to the rule of law.

Nixon, for instance, sought to compromise U.S. District Judge Matthew Byrne, who, at the time, was presiding over the criminal trial of Daniel Ellsberg for leaking the Pentagon Papers to The New York Times and The Washington Post.

Nixon had much to fear from potential disclosures in that trial because members of the so-called plumbers unit in the White House, under the direction of Howard Hunt and G. Gordon Liddy (later involved in the Watergate scandal), had burglarized the office of Ellsberg's former psychiatrist, Dr. Lewis Fielding, looking for dirt on Ellsberg, whose telephone had also been tapped without authorization by the FBI.

But even those actions were not enough to satisfy Nixon. According to the House Judiciary Committee's final report in the Nixon impeachment inquiry: "The president later directed John Ehrlichman to contact Judge Matthew Byrne, the presiding judge in the Ellsberg trial. On April 5 and 7, 1973 Ehrlichman met with Judge Byrne and informed him that the president was considering appointing Judge Byrne to the directorship of the FBI."

Byrne said he spurned the overture, and when prosecutors (in a sealed filing) told Byrne about the Fielding burglary, he ordered the information to be released. Two days later, in an effort to embarrass the judge, the White House leaked that Byrne had met with White House officials about being offered the FBI post.

Byrne refused to recuse himself from the case and subsequently declared a mistrial because of governmental misconduct, both freeing Ellsberg and a co-defendant and barring their future prosecution.

Similarly, both Trump and Nixon tried to interfere with their own Justice Departments' investigations of close advisers.

In April 1973, Nixon received private briefings from Attorney General Richard Kleindeinst and the assistant attorney general in the Criminal Division, Henry Petersen, about the Watergate investigation of Nixon's aides. Nixon pledged to Petersen that he would not share the grand jury information with anyone. Nixon broke that promise immediately, passing along confidential information to the very aides under investigation.

Trump's transgressions are more public, made in Twitter blasts to millions of followers. For instance, he tweeted last week complaining about the prosecutors' sentencing recommendation (within guidelines) of seven to nine years in the Stone case, and less than a day later, the Justice Department recanted its recommendation as too severe. That led to the very public resignations of four career prosecutors from the case and a public outcry about Trump's undermining of the rule of law.

The blowback even led Attorney General William Barr, whom Trump had congratulated for rescinding the first sentencing memorandum, to say in an interview Thursday, "I think it's time to stop the tweeting about Department of Justice criminal cases." (Trump, of course, did no such thing.)

But Trump's Twitter-ference in the Stone case is only the latest example of his public meddling in what would normally be matters confined to the Justice Department: He has previously demanded — via tweets — that the Justice Department investigate Hillary Clinton and had those requests obeyed, as well as whether his 2016 political campaign was surveilled or infiltrated by the FBI and had that request fulfilled.

But the results of the two presidents' attempts diverge even more sharply than that: Nixon was impeached for obstruction of the Justice Department investigation. The Senate acquittal in Trump's impeachment trial has seemingly given Trump a green light to continue to intimidate prosecutors and judges in various, often related, criminal cases.

Still, just as Nixon was ultimately unsuccessful in sabotaging judicial independence, Trump may be, too — if his actions alienate an important audience of one: Roberts.

Although the chief justice sat impassively as House managers presented the evidence supporting the abuse of power article in the Senate impeachment trial, he heard it all. He assuredly observed the pattern showing Trump's willingness — or, really, eagerness — to demolish the norms and limitations on his power as president.

And now, on March 31, the Supreme Court will hear oral arguments over whether Trump's accountants and bankers must (as lower courts have ruled) turn over Trump's tax returns and financial records in response to subpoenas issued by the House of Representatives and a New York grand jury.

No one would suggest that Roberts should or would rule against Trump in response to the president's attacks on the federal judiciary. But it is a fact that the president's lawyers will be asking the Supreme Court to create a broad protection for a president who already claims to be (and acts as though he is) excused from the legal obligations imposed on everyone else.

Against that background, the Supreme Court should reaffirm the separation of powers and compel Trump to comply with congressional oversight and grand jury demands. Maybe then Trump will learn, at a minimum, not to attack federal judges who enjoy lifetime tenure.