President Barack Obama speaks at the Presidential Summit of the Mandela Washington Fellowship for Young African Leaders at the Omni Shoreham Hotel August 3, 2016 in Washington, DC.
BRENDAN SMIALOWSKI/AFP/Getty Images
President Barack Obama speaks at the Presidential Summit of the Mandela Washington Fellowship for Young African Leaders at the Omni Shoreham Hotel August 3, 2016 in Washington, DC.
Now playing
02:29
Obama takes on Trump with tough talk
Green Party presidential nominee Jill Stein waits to be introduced prior to a press conference at the National Press Club August 23, 2016 in Washington, DC.
Win McNamee/Getty Images
Green Party presidential nominee Jill Stein waits to be introduced prior to a press conference at the National Press Club August 23, 2016 in Washington, DC.
Now playing
01:44
Jill Stein raises millions for recount
Former Democratic US Presidential candidate Hillary Clinton speaks to staff and supporters at the New Yorker hotel after her defeat in the presidential election November 9, 2016 in New York. / AFP / Brendan Smialowski        (Photo credit should read BRENDAN SMIALOWSKI/AFP/Getty Images)
BRENDAN SMIALOWSKI/AFP/Getty Images
Former Democratic US Presidential candidate Hillary Clinton speaks to staff and supporters at the New Yorker hotel after her defeat in the presidential election November 9, 2016 in New York. / AFP / Brendan Smialowski (Photo credit should read BRENDAN SMIALOWSKI/AFP/Getty Images)
Now playing
02:14
Scientists urge Clinton to call for recount
LYNDEN, WA - MAY 07: Republican presidential candidate Donald Trump gives a speech during a rally at the The Northwest Washington Fair and Event Center on May 7, 2016 in Lynden, Washington. Trump became the Republican presumptive nominee following his landslide win in Indiana on Tuesday. (Photo by Matt Mills McKnight/Getty Images)
Matt Mills McKnight/Getty Images North America/Getty Images
LYNDEN, WA - MAY 07: Republican presidential candidate Donald Trump gives a speech during a rally at the The Northwest Washington Fair and Event Center on May 7, 2016 in Lynden, Washington. Trump became the Republican presumptive nominee following his landslide win in Indiana on Tuesday. (Photo by Matt Mills McKnight/Getty Images)
Now playing
03:25
Donald Trump's wild ride
Now playing
01:46
Trump's challenges in the Middle East
donald trump undocumented immigrants crime fact check origwx ty_00013807.jpg
donald trump undocumented immigrants crime fact check origwx ty_00013807.jpg
Now playing
02:47
Fact checking Trump on crimes by immigrants
Mark Makela/Getty Images/Lintao Zhang/Getty Images
Now playing
01:26
Trump vs. the tape on Obama and the protester
Now playing
01:28
Trump compares Clinton email probe to Watergate
Members of the Secret Service rush Republican presidential candidate Donald Trump off the stage at a campaign rally in Reno, Nev., on Saturday, Nov. 5, 2016. (AP Photo/John Locher)
John Locher/AP
Members of the Secret Service rush Republican presidential candidate Donald Trump off the stage at a campaign rally in Reno, Nev., on Saturday, Nov. 5, 2016. (AP Photo/John Locher)
Now playing
01:20
Donald Trump rushed off stage by Secret Service
AFP/Getty Images
Now playing
02:16
Trump reads letter from Patriot's coach at rally
US Republican presidential nominee Donald Trump speaks during rally at the Atkinson Country Club in Atkinson, New Hampshire on November 4, 2016.
Mandel Ngan/AFP/Getty Images
US Republican presidential nominee Donald Trump speaks during rally at the Atkinson Country Club in Atkinson, New Hampshire on November 4, 2016.
Now playing
01:56
Trump on US officials: 'What a group of losers'
donald trump nice and cool pensacola fl bts_00001523.jpg
donald trump nice and cool pensacola fl bts_00001523.jpg
Now playing
00:49
Trump's mantra in campaign's final week: Stay on point
donald trump nice to hillary clinton and democrats orig cm_00000000.jpg
donald trump nice to hillary clinton and democrats orig cm_00000000.jpg
Now playing
01:52
Audio of Trump praising the Clintons and Democrats
cuomo lewandowski hillary clinton email exchange newday_00002327.jpg
cuomo lewandowski hillary clinton email exchange newday_00002327.jpg
Now playing
01:28
Cuomo grills Lewandowski on Clinton email 'hypocrisy'
Now playing
00:10
This video is no longer available
Now playing
01:49
Meet the other Hillary Clinton and Donald Trump

Story highlights

Obama says Trump is temperamentally and intellectually unfit for the presidency

His attacks set him apart from almost all of his 43 predecessors

Washington CNN —  

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.

RELATED: Obama: Trump’s warning on elections is ‘ridiculous’

“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.

RELATED: Donald Trump’s history of suggesting Obama is a Muslim

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.

RLEATED: Can a rising Obama help Hillary Clinton?

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.

RELATED: Trump responds to Obama: ‘He’s a terrible president’

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.