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.

Bad personal history

And it was the billionaire who first made a target of Obama, spending years trying to prove the Hawaii native was not born in the United States. It was an assault -- tied to Obama's father's Kenyan heritage -- that the President's allies saw as racially motivated.

But this is beyond personal. Buried in Obama's speech at the Democratic National Convention last month in Philadelphia was the implicit suggestion that Trump was a "homegrown" demagogue who threatened American values just as fascists, communists and jihadists do.

He reinforced the point on Tuesday at a White House news conference . "I think the Republican nominee is unfit to serve as president. I said so last week, and he keeps on proving it," Obama said, criticizing Trump for lacking knowledge on key questions in Asia, Europe and the Middle East.

On Thursday, Obama questioned whether the billionaire can be trusted with his finger on the button.

"Just listen to what Mr. Trump has to say and make your own judgment with respect to how confident you feel about his ability to manage things like our nuclear triad," Obama said in response to a question from CNN's Barbara Starr.

The President is also trying to convince Americans that his antipathy towards the GOP nominee is about more than politics, mirroring Clinton's own efforts to reach out to Republicans alienated by Trump.

"I think I was right -- and Mitt Romney and John McCain were wrong -- on certain policy issues, but I never thought that they couldn't do the job," he said Tuesday.

Obama has some political cover, because his complaints about Trump are part of a wider critique among elite opinion makers about the billionaire.

The former acting director of the CIA, Michael Morell, hardly known as a political partisan, on Friday published an op-ed in The New York Times suggesting Trump had become an unwitting agent of the former KGB agent in the Kremlin, President Vladimir Putin.

The Clinton campaign, meanwhile, has been for weeks trying to build a case that Trump would pose an unacceptable risk as commander in chief -- and the Republican nominee is not always helping himself.

In the last 10 days, he's appeared to invite Russian intelligence agents to hack the US government; he feuded publicly with the parents of fallen Muslim US soldier Capt. Humayun Khan; and statements he made about Ukraine and nuclear doctrine raised questions about his grasp on the current state of both topics.

Obama, for his part, is enjoying some of the best approval ratings of his second term and is in a unique historical position -- though it remains to be seen to what extent his politicking helps Clinton.

Still, since the passage of the 22nd Amendment limiting a president's time in office to two elected terms was ratified in 1951, Obama is in perhaps the best political shape of any second-term incumbent.

Dwight D. Eisenhower cut back on his political activity due to advancing age and was of limited help to Vice President Richard Nixon in the 1960 campaign. Reagan was hobbled by the Iran-Contra affair and lacked the vigor of Obama despite his campaign trail appearances for Bush. President Bill Clinton was itching to play in the 2000 election but his impeachment led Vice President Al Gore to keep his distance. And President George W. Bush, brought low by the Iraq War, Hurricane Katrina and the financial crisis, was a millstone for 2008 GOP nominee McCain.

Perhaps because of his dinged popularity, Bush took the opposite approach to Obama.

"I will resist all temptation to become the pundit in chief and commenting upon every twist and turn of the presidential campaign," he told reporters asking his thoughts on the upcoming election at a 2007 White House news conference.

Obama effect could linger

Although Obama will leave the White House on January 20, the implications of his interventions could linger much longer.

Once a precedent is broken, it becomes easier for a future president to behave in the same way.

With that in mind, Smith said that Obama's remarks could be seen as unpresidential unless he sincerely believed that Trump was a danger to the nation.

"I think he does, and, of course, he is not alone," Smith said.

Since many second-term presidents limp toward the exit, however, the chances that Obama's maneuvering could become routine for presidents are low, according to Zelizer.

"The check against this will be the same thing that checked it in the past," he said. "Often the person in the White House has baggage or is no longer popular."

Obama's aggressive attacks could also have more immediate political reverberations.

Should Trump win the presidency, he will take office after the nation has been told he is not fit for the job by someone who should know.

Such a scenario would all but ensure a deepening of already raw political divides and could make governing nearly impossible.

But Obama is willing to take that risk. Even though he's not looking to take daily shots at Trump, he will seek new opportunities to land political blows, a White House source told CNN's Michelle Kosinski on Friday.

Aides believe that the president can dictate news headlines just by speaking and has the unique perspective of having served in the White House. Most of all, he feels a responsibility to counter Trump's message, the source said.

In the end, though, Obama realizes that should the GOP nominee reverse the damage of his rocky political week and win the election, he will have no choice but to do his duty.

"If somebody wins the election and they are president, then my responsibility is to peacefully transfer power to that individual and do everything I can to help them succeed," he said on Thursday.