Julian Zelizer is a history and public affairs professor at Princeton University and the author of " The Fierce Urgency of Now: Lyndon Johnson, Congress, and the Battle for the Great Society. " He's also the co-host of the " Politics & Polls " podcast. The opinions expressed in this commentary are his own.

(CNN) During a conversation with David Letterman on his new Netflix program, former President Barack Obama issued a stern warning for the current commander in chief: "One of the things that Michelle figured out, in some ways faster than I did, was part of your ability to lead the country doesn't have to do with legislation, doesn't have to do with regulations, it has to do with shaping attitudes, shaping culture, increasing awareness." Although Obama was extremely hesitant about directly commenting about President Donald Trump, the message he sent was clear.

And it comes in the wake of the upsetting news that during a private meeting with legislators at the White House, President Trump referred to African nations as "shithole countries." The President tweeted out a denial that he ever said this, with two Republican legislators saying that they couldn't remember. Illinois Democrat Richard Durbin, who was in attendance, responded that Trump did use the term and said things that were "hate-filled, vile and racist." And a Republican, Sen. Lindsey Graham of South Carolina, said he appreciated Durbin's statement -- and had challenged Trump's comments.

It would be a mistake to dismiss the comments as a "distraction" because, with Trump, they are the main show. His rhetoric sends a message to the nation and to the world about the values that we treasure and that the nation will stand for in 2018.

As we learned again this week, the President is willing to use crass and hateful rhetoric in public and in formal events in a way we have not seen in the contemporary era, or at least not from presidents who were acting in the way we hope to see. To be sure, Americans understand that their presidents are very human and can speak in familiar ways when they are in private. They curse, they scream, they yell and sometimes they say mean things.

Speaking to a military adviser, President Lyndon Johnson called Vietnam a "little pissant country" and in racist phone conversations he can be heard uttering racist rhetoric. The architect of the Civil Rights Act of 1964 and Voting Rights Act of 1965 was not shy about using the "N Word" when talking with southerners, or comparing African-Americans to ungrateful children after the riots of 1965 and 1967.

Following the Watts riots in 1965, Johnson complained to the president of the Steelworkers Union that African Americans had to learn they have "obligations as well as rights." They needed to be more "responsible." Much of his daily conversation was too R-rated for children. "I do know the difference between chicken shit and chicken salad," he reportedly said.

Baby Boomers will remember just how shocked they were during the Watergate investigation to learn through the transcripts of the White House recordings that President Richard Nixon swore, personally insulted his enemies, and used anti-Semitic language behind closed doors. Nixon referred to Henry Kissinger as "Jew-boy."

Foul language in the White House didn't end there. Talking about a primary challenge from Sen.Ted Kennedy, President Carter famously told a group of legislators that he would "whip his ass" while President Clinton was famous for his curse-filled tirades in private meetings

But even after the famous "expletives-deleted" transcripts from the Nixon investigation, the country expected that, in public and in formal political events, presidents would abide by a certain level of decorum and refrain from using harsh expletives. Sure, some presidents were caught cussing on a hot mic (such as when George W. Bush was heard calling reporter Adam Clymer a "major league asshole"), but those were exceptions.

The hope for many Americans was also that presidents would stop using the kind of demeaning language heard in the White House tapes of the 1960s and early 1970s -- that was the lesson of the "Expletives Deleted" controversy. Being presidential meant no longer engaging this language in public or even private settings, within reason, in the aftermath of the civil rights and feminist movements.

This administration has not fulfilled those hopes. The problem really isn't Trump's use of curse words, but the words he chose to use and the context within which they were uttered.

President Trump, who launched his campaign by calling Mexican immigrants rapists and criminals, was reported to have said that all Haitians "have AIDS" (the White House denied the statement) and that Nigerians "live in huts." When he said during the campaign that US Judge Gonzalo Curiel should recuse himself from the Trump University lawsuit because of his Mexican background, Speaker Paul Ryan called it the "textbook definition of a racist comment." Trump's comments about Muslims in the US and around the world, perpetually painting them as a dangerous and hostile population, are too extensive to list in one article.

Trump has repeatedly made disparaging comments about women, from Rosie O'Donnell to Hillary Clinton to Mika Brzezinski. His refusal to come down hard against the Nazis in Charlottesville -- saying that "both sides" were to blame -- was in itself a powerful speech act loaded with racial implications, all of which builds on a long and controversial history where he has been accused of racism.

Trump insists on calling Sen. Elizabeth Warren "Pocahontas" and has gone after football players protesting police violence against African-Americans. Soon after Puerto Ricans suffered through a devastating hurricane, his instincts led him to tweet out that the inhabitants "want everything to be done for them when it should be a community effort." So much for compassion.

Just last August, he retweeted a meme showing him "eclipsing" former President Obama, sharing a post that came from someone on Twitter who posted anti-Semitic remarks. This, from a person who made his national political name by challenging the birthplace of the first African-American President, did not sit well with many people.

As David Leonhardt argued in the New York Times in his troubling catalog of the President's long history with racism, "He has retweeted white nationalists without apology. He frequently criticizes prominent African Americans for being unpatriotic, ungrateful and disrespectful. He called some of those who marched alongside white supremacists in Charlottesville last August 'very fine people.' He is quick to highlight crimes committed by dark-skinned people, sometimes exaggerating or lying about it."

The hope was that presidents would elevate our national discourse, not debase it. This has not been the case with President Trump, who has bombarded the country with a level of vile rhetoric that the nation has not witnessed before. The fact that he used curse words is just a minor part of the problem. There have not been any angels in the Oval Office. If cursing was the only issue, the response would not have been so explosive; nor would his actions have been so exceptional.

Much more troubling is the kind of hateful invective that these curse words, which were grounded in both racist and nativist sentiment, express against social groups whom the President has repeatedly targeted.

In using this kind of rhetoric, Trump has been "unpresidential" in that he has allowed the words and ideas from reactionary extremist groups, that defend social inequality and promote hatred, to enter into the highest levels of power.

We have seen these moments so many times since the 2016 campaign it becomes impossible to dismiss them as mistakes or aberrations. Even if the President continues to dispute the words he used in this particular private meeting, there is a long list of harsh statements that come directly out of the far-right universe.

In 2018, echoing anything that these groups say should be, by definition, unpresidential. The job of the president is to push back against these elements of society, even if they lean toward his or her own political coalition, to remain an example of the direction in which the country should move.

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With all the recent obsession with his "mental fitness," it is crucial to avoid allowing that conversation to downplay the very intentional words that he has used to inflame social tensions and fuel irrational anger, fulfilling his promise of populism by directing anger toward certain groups rather than offering economic relief.

President Trump's rhetoric is also unpresidential in that he is willing to do and say big things, that entail huge risks for the country, in a dangerously ad hoc manner. Nowhere has this been clearer than with his tweets about North Korea.

Part of the job of being president, and particularly of being a good one, is to remain deliberative and to act with extraordinary caution based on the realization that every statement can trigger massive consequences, some even deadly. Most presidents know this upon taking the job and the rest learn it very quickly as they confront the realities of governance.

Obviously, many presidents have been willing to act by their gut and sometimes go off-script. This can lead to good things. But this is different. This is about a president apparently acting this way much of time, unless forced to contain himself when the cameras are rolling in a staged conversation with legislators and who lacks the basic respect for the institution he now controls to take norms seriously.

To say that Donald Trump is not acting presidential is not to romanticize what we have seen from previous inhabitants of the office. But it is to hold him accountable for going far beyond the proper limits on presidential behavior.

The biggest danger is that by tolerating Trump's behavior in office, the public will make what he is doing and saying part of our conception of what it means to be presidential.

It is vital that members of both parties admit what they see when these moments happen and avoid normalizing these kinds of reckless departures from presidential history. For if the political class, and the public, starts to brush these moments off as "Trump being Trump" or "nothing worse than what we have seen" we will lower the bar so far it will be impossible to ever repair the presidency.