In Dallas he was prepared to decry, “voices preaching doctrines wholly unrelated to reality,” which he feared could, “handicap this country’s security.”

He planned to say that “We cannot expect that everyone, to use the phrase of a decade ago, will ‘talk sense to the American people.’ But we can hope that fewer people will listen to nonsense.”

It was to have been a bold statement and a sharp warning, one that might have altered to contours of our national response to today’s violent, disassociated rhetoric — had he lived to deliver it.

We often search leaders’ last words for deeper meaning, a message to the ages. Although Thomas Jefferson’s last words were to his servants in the early-morning hours of July 4, 1826, and went unrecorded, and his last recorded words were to his physician, “No doctor, nothing more,” we instead focus on the fact that on the evening of July 3, Jefferson woke and asked with insistence, “Is it the Fourth?” It seems more appropriate that Jefferson’s last words ask about the independence movement he helped set in motion exactly 50 years earlier. Of course, the reason we often take poetic license with last words is that people very rarely know what their last words will be, especially when death arrives unexpectedly.

Regardless of whether the speaker knew that death was near or not, we ascribe to those final statements a weight that we might not otherwise. Perhaps it’s because we never got to hear those words delivered in life that we hear them more clearly in death.

The final chapter of my new book, “Undelivered,” which covers roughly 20 historically significant undelivered speeches, looks at the speeches that Pope Pius XI, Franklin Delano Roosevelt, Albert Einstein, and John F. Kennedy were working on at the time they died.

Each has a powerful message to a future they wouldn’t live to see.

As 1962 became 1963, President John F. Kennedy was enormously popular. Having successfully navigated the Cuban missile crisis, he began the year with a 70 percent approval rating. In March, he held a 67 percent to 27 percent polling advantage over the leading Republican challenger, Arizona Senator Barry Goldwater. Kennedy was also a cultural phenomenon; roughly half of all Americans had seen or heard a Kennedy imitator. But, as the year progressed, Kennedy’s focus on civil rights began to take a toll. His popularity dipped overall, and nose-dived in the South.

By the time Air Force One landed in Dallas, Texas had become an important political battleground. As The New York Times reported in early November, “Even if Mr. Kennedy should write off most of the South, he is not writing off Texas’s 25 votes.”

Looking at the documents that various administration and DNC officials submitted to speechwriter Ted Sorensen in order to prepare for the trip, one sees familiar building blocks to anyone trying to make a political argument today: a political update memo from the Democratic National Committee, an article on the economic situation in Texas from the Texas Business Review, and “administration accomplishments” documents for Texas that included statistics on public works spending, small business aid, and oil and gas leasing progress. Sorensen also assembled a collection of “Texas humor” Kennedy had requested.

In Dallas, Kennedy was prepared to speak to an audience comprised of several different groups. It included members of the Dallas Citizens Council and the Dallas Assembly, two groups of local business and nonprofit leaders, with another contingent from the Graduate Research Center of the Southwest. For speechwriters, audiences like this can pose a challenge: How do you address your remarks to all the groups while saying something meaningful to each?

Kennedy found his unifying theme in the “link between leadership and learning.” In words Kennedy was to deliver: “leadership and learning are indispensable to each other. The advancement of learning depends on community leadership for financial and political support and the products of that learning, in turn, are essential to the leadership’s hopes for continued progress and prosperity.”

While the previous speeches Kennedy had given and those he planned to give on his Texas trip were generally workmanlike, including lists of accomplishments and solicitations of support, Kennedy was prepared to take a different approach with this speech and audience. For starters, it leaned heavily on national security.

To the extent Kennedy’s last, undelivered words are remembered, it is because of the powerful and well-publicized conclusion. Kennedy planned on ending his speech with these words:

We in this country, in this generation, are—by destiny rather than choice—the watchmen on the walls of world freedom. We ask, therefore, that we may be worthy of our power and responsibility, that we may exercise our strength with wisdom and restraint, and that we may achieve in our time and for all time the ancient vision of “peace on earth, good will toward men.” That must always be our goal, and the righteousness of our cause must always underlie our strength. For as was written long ago: “except the Lord keep the city, the watchman waketh but in vain.”

Yes, Kennedy wanted America to serve as the watchman on the wall for world freedom. But a closer reading of the speech and the circumstances in which it was drafted shows that Kennedy recognized and wanted to make clear that the watchman on the wall can’t just look outward to see threats to freedom — he must also look inside the walls as well.

Although the speech is remembered as one devoted to national security, nearly half is devoted to a different concern: what Sorensen described as “the fires of rage” that burned beneath the surface of America’s peace and prosperity.

These fires of rage revealed themselves in an increasingly vocal right-wing effort to discredit and demonize Kennedy. One of the leaders of this effort was Edwin Walker, a former World War II general who helped foment riots at the University of Mississippi when the school attempted to integrate by admitting James Meredith in 1962.

Walker also ran as a fringe candidate for governor of Texas. Using language similar to the attacks President Donald Trump and his supporters would wage on his political opponents a half century later, Walker declared that civil rights demonstrations in Washington and Texas were “pro-Kennedy, pro-Communist and pro-Socialist.”

As we have seen all too often recently, violent words are often a precursor — and provide a permission structure — for violent actions.

In fact, a month earlier in Dallas, remarks by Adlai Stevenson were disrupted by Walker supporters who held American flags upside down (a tactic Walker encouraged), unfurled a banner that replaced the words Welcome Adlai with UN RED FRONT, and tried to drown out Stevenson’s words with noisemakers.

The scene is recounted in masterful and harrowing detail in the book Dallas 1963 by Bill Minutaglio and Steven L. Davis.

As one particularly combative heckler was escorted out, Stevenson called after him: “For my part, I believe in the forgiveness of sin and the redemption of ignorance.”

It was an unintentional precursor to Kennedy’s language about the linkage between leadership and learning.

After the remarks, Stevenson was spit on and one protestor, Cora Lacy Frederickson, began hitting Stevenson with large sign. It read: ADLAI, WHO ELECTED YOU?

In advance of Kennedy’s arrival, Walker and his followers felt further emboldened. They distributed leaflets accusing Kennedy of treason, of being “lax” on communism, and of “appointing anti-Christians to Federal office.”

Indeed, this is why Kennedy opened his address with a statement about the importance of learning, hoping to remind people to tether attitudes and opinions to facts.

America’s leadership must be guided by the lights of learning and reason or else those who confuse rhetoric with reality and the plausible with the possible will gain the popular ascendancy with their seemingly swift and simple solutions to every world problem.

If Kennedy wanted to talk about the important linkage between leadership and learning, he also wanted to remind people how much leadership can be sacrificed when we turn away from learning and embrace ignorance.

And so Kennedy sought to address this dangerous, angry, violence-inducing disassociation from reality in his address. He distinguished this kind of attitude from the constant complainers, the “dissident voices” who will always be “expressing opposition without alternatives, finding fault but never favor, perceiving gloom on every side and seeking influence without responsibility.”

Those voices of constant complaint, Kennedy was to say, “are inevitable.” While he accepted the inevitability of dissident voices, he was more concerned about those who knowingly promote and spread lies.

But today other voices are heard in the land — voices preaching doctrines wholly unrelated to reality, wholly unsuited to the sixties, doctrines which apparently assume that words will suffice without weapons, that vituperation is as good as victory and that peace is a sign of weakness. At a time when the national debt is steadily being reduced in terms of its burden on our economy, they see that debt as the greatest single threat to our security. At a time when we are steadily reducing the number of Federal employees serving every thousand citizens, they fear those supposed hordes of civil servants far more than the actual hordes of opposing armies.

Update hordes of immigrants for hordes of civil servants (the point about the debt, ironically, remains as accurate and relevant now as it did then), and this warning resonates clearly today.

Ignorance and misinformation can handicap the progress of a city or a company, but they can, if allowed to prevail in foreign policy, handicap this country’s security.

Kennedy’s hope? “We cannot expect that everyone, to use the phrase of a decade ago, will ‘talk sense to the American people.’ But we can hope that fewer people will listen to nonsense.”

Leaders play a role in increasing our awareness of threats and conditioning our responses to them.

Would Americans have sat up and paid attention if their president had hectored them to stop listening to nonsense?

If Kennedy had lived and secured a second term, would he have made combatting domestic extremism a priority — articulating the threat from within as clearly as clearly as he articulated the threat from Russia in his first campaign?

Of course, we cannot know.

But what we do know is that Kennedy wanted an America with fewer people listening to and falling prey to nonsense. In his unspoken last speech, Kennedy left us with a warning against the type of angry, disassociated rhetoric that is causing such damage to, and within, democratic governments around the world today.