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By PETER BAKER

WASHINGTON — When President Trump meets with aides to discuss policy or prepare for a speech, he may ask about the pros and cons of a new proposal. He may inquire about its possible effect. He may explore the best way to frame his case.

But there is one thing he almost never does. “He very seldom asks how other presidents did this,” said John F. Kelly, the White House chief of staff.

Mr. Trump is the 45th president of the United States, but he has spent much of his first year in office defying the conventions and norms established by the previous 44, and transforming the presidency in ways that were once unimaginable.

Under Mr. Trump, it has become a blunt instrument to advance personal, policy and political goals. He has revolutionized the way presidents deal with the world beyond 1600 Pennsylvania Avenue, dispensing with the carefully modulated messaging of past chief executives in favor of no-holds-barred, crystal-breaking, us-against-them, damn-the-consequences blasts borne out of gut and grievance.

He has kept a business on the side; attacked the F.B.I., C.I.A. and other institutions he oversees; threatened to use his power against rivals; and waged war against members of his own party and even his own cabinet. He fired the man investigating his campaign and has not ruled out firing the one who took over. He has appealed to base instincts on race, religion and gender as no president has in generations. And he has rattled the nuclear saber more bombastically than it has been since the days of Hiroshima and Nagasaki.

The presidency has served as a vehicle for Mr. Trump to construct and promote his own narrative, one with crackling verve but riddled with inaccuracies, distortions and outright lies, according to fact checkers. Rather than a force for unity or a calming voice in turbulent times, the presidency now is another weapon in a permanent campaign of divisiveness. Democrats and many establishment Republicans worry that Mr. Trump has squandered the moral authority of the office.

“We’re seeing the presidency completely and utterly transformed in a way I don’t think we’ve seen since before the Civil War,” said Jeffrey A. Engel, the director of the Center for Presidential History at 91��Ƭ�S�ϼ� and the author of “When the World Seemed New” about President George Bush. “Trump is arguing that we need to take care of my enemies. I really can’t think of any precedent.”

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