For decades American presidents have carefully planned their public utterances. Donald J. Trump’s improvisational, even impulsive, style has changed that paradigm.

For decades American presidents have embraced a combination of international engagement and free trade. Trump has upended that.

For decades American presidents have courted established power centers, including the news media. Trump has turned that Washington fandango on its ear.

Both innovator and disrupter, Trump in his 16 months in the White House has challenged presidential customs, overhauled presidential expectations and reshaped popular notions of presidential style. And he has done one thing more:

He has obliterated the decades-old taboo of demonizing his predecessors.

In all of the modern party transitions in the White House — William Howard Taft to Woodrow Wilson and then to Warren Harding, Herbert Hoover to Franklin Roosevelt, Harry Truman to Dwight Eisenhower to John F. Kennedy, Lyndon Johnson to Richard Nixon — new presidents generally have refrained from attacking their predecessors.

Jimmy Carter moved close to Gerald Ford, Ronald Reagan treated Carter with deference, Bill Clinton was so attentive to George H.W. Bush that his successor, George W. Bush, complained that Clinton was nudging out the Bush children for their father’s affection.

But Trump has made a habit of criticizing his predecessor, Barack Obama, who only this month struck back, defending the Iran nuclear treaty that Trump described as “this disastrous deal” and “a great embarrassment to me as a citizen and to all citizens of the United States.”

Trump has even suggested he could have forestalled the Civil War, a task one of his Republican predecessors, Abraham Lincoln, was unable to perform.

Some historians, who generally dismiss the notion of historical inevitability, nonetheless believe the 1861-65 war was, as Sen. William Seward, a leading figure of the time, put it in 1858, an “irrepressible conflict.”

By some lights — indeed, by the testimony of Kennedy — Trump has a right to engage in such historical criticism. “No one,” Kennedy said, “has a right to grade a president — not even poor James Buchanan — who has not sat in his chair, examined the mail and information that came across his desk, and learned why he made his decisions.”

Even so, Trump’s critiques are a departure from presidential form, perhaps welcome to his supporters, surely jarring to others. Indeed, Trump is a president without precedent, and is disinclined to seek them.

“Presidents often cite other leaders for a model, and he never does that,” says Pierre Martin, an expert in American politics who teaches at the University of Montreal. “His main opposition to the Iran agreement is that someone else made the deal. His major objection is that he wasn’t involved in it.”

Earlier presidents generally showed deference to their predecessors; though Kennedy’s 1960 campaign was based on a theme implicitly ridiculing Eisenhower — “Let’s get America moving again” — the new president sought the advice of his predecessor during foreign-policy crises in his presidency.

Often these hands across the generations and across party lines begin with small gestures. Kennedy, for example, supported Eisenhower’s request that his five-star rank as general of the Army be restored so in retirement he would be known as Gen. Eisenhower rather than as Mr. President. When Kennedy died, Truman and Eisenhower, whose frosty ride to the 1953 inauguration was the stuff of legends, shared a whiskey at Blair House before they rode together to Arlington National Cemetery.

Though presidents generally have avoided criticizing their predecessors, former presidents often have been fiercely critical of their successors.

In a new biography of the 31st president, Kenneth Whyte spoke of Hoover’s “hatred of the new president,” explaining that he believed “the New Deal’s scale, reach and coercive nature were so far beyond anything America had known as to represent a new and dangerous doctrine in national political life.” Later Hoover criticized Roosevelt for extending Lend-Lease to the Soviet Union, though he pledged support for FDR after Pearl Harbor.

While the Eisenhower-Kennedy relationship was respectful, the general criticized the “careless spending” of the Kennedy team and was wary of the administration’s military buildup. But, as George W. Bush liked to say, we can only have one president at a time.

Well, maybe more than one, if a riff by Reagan is to be believed.

Listen in for just a moment to Reagan’s account of the White House when the tourists and aides have gone home:

“Nothing is ever lost in that great house; some music plays on. I have been told that late at night when the clouds are still and the moon is high, you can just about hear the sound of certain memories brushing by. You can almost hear, if you listen close, the whir of a wheelchair rolling by. … Turn down a hall and you can hear the brisk strut of a fellow saying, ‘Bully! Absolutely ripping!’ Walk softly now and you’re drawn to the soft notes of a piano and a brilliant gathering in the East Room, where a crowd surrounds a bright young president who is full of hope and laughter. I don’t know if this is true … but it’s a story I’ve been told.”

Maybe Trump should listen for that story, too. His predecessors may be out of office, but they retain enormous power — the power, Trump may discover, to inspire, or to comfort.

David M. Shribman is executive editor of the Post-Gazette (dshribman@post-gazette.com, 412 263-1890). Follow him on Twitter at ShribmanPG.