It’s one more historic barrier President Barack Obama has shattered.

His vehement warnings that GOP nominee Donald Trump is temperamentally and intellectually unfit for the Oval Office leave Obama standing apart from almost all of his 43 predecessors in the extent to which he has publicly expressed a hostile attitude to a potential successor.

During yet another turbulent week in a convention-busting election campaign, Obama cloaked himself in the symbolism-laden settings of the Pentagon and an appearance with a foreign dignitary in the White House to denounce Trump as “unfit” for the Oval Office.

His intent was not merely to stage a political intervention to improve the election chances of fellow Democrat Hillary Clinton and with it the likelihood of securing his legacy.

He was also delivering a warning to the American people: Think very seriously about handing Trump the national jewel of the presidency and all it represents in international standing and nuclearized military power.

Tensions between presidents and potential successors are not new. Other commanders in chief have strongly backed a preferred successor. Ronald Reagan, for instance, worked hard to pass the baton of power to his vice president, George H.W. Bush, in 1988.

But Obama’s withering dismissal of the opposing party’s nominee in such explicit terms is unique in the modern presidency, historians say.

“This is as aggressive as we have seen. (Obama) is the strongest president in recent decades in terms of intervening in the campaign,” said Julian Zelizer, a professor of history at Princeton University. “Not only is he active; he is making incredibly tough statements.”

Robert Smith, a professor of political science at San Francisco State University, agreed: “Obama’s remarks are unprecedented in modern times for sure.”

Smith said the only parallel to Obama’s stance on Trump were warnings by President John Quincy Adams about his eventual successor Andrew Jackson, who was decried by the 1820s Eastern establishment as an uncouth outsider prone to cursing and womanizing.

Obama’s repudiation of Trump goes way beyond traditional doubts that presidents often have about the capacity of successors to do their job well.

“It is unprecedented in recent American history the way President Obama has been lambasting Trump as being a dangerous menace to America,” said CNN presidential historian Douglas Brinkley of Rice University.

“Obviously, there are times in the 19th century around the Civil War where tensions were high and name-calling was prevalent. But in modern times,” he said, “we have never seen such a spectacle.”

Obama has never hidden his disdain for Trump and a style of politics that he sees as frivolous, divisive and lacking gravitas.

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