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Editor’s Note: Julian Zelizer is a history and public affairs professor at Princeton University and co-host of the “Politics & Polls” podcast. In January, Norton will publish a new book with Kevin Kruse, “Fault Lines: A History of the United States Since 1974.” Follow him on Twitter: @julianzelizer. The opinions expressed in this commentary are those of the author. View more opinion articles on CNN.

(CNN) —  

It turns out that the caravan Americans should be worried about is already here. It is the white van that the alleged, attempted mail bomber Cesar Sayoc was driving. Fortunately, federal authorities were able to arrest and charge him with sending explosive devices to the homes and offices of many of President Donald Trump’s top opponents, to two former Presidents, and to two former intelligence officials, care of CNN’s New York offices.

In a bid to whip up the enthusiasm of the Republican base with the midterm elections looming, Trump has tried to paint a march of protesting people traveling through Mexico, who are weeks away from the US border, as an imminent threat to America’s safety. But his narrative was interrupted rudely this week by a much more tangible threat – the discovery of explosive devices sent from inside the US homeland.

Although it will probably take some time before the FBI can prove whether Sayoc was guilty and whether he acted alone, the arrest is a troubling reminder of the ways in which the President’s toxic political ideas have become sources of inspiration for extremist individuals and organizations – even those who want to commit violence.

With all the attention that the President has devoted in the past few weeks to the allegedly dangerous people who are part of the movement of immigrants seeking safety within our borders, a real concern for the country should be the potential for violent domestic political extremism to flare among people who live here and who perceive themselves to have an ally in the White House.

Domestic political violence in the United States is no joking matter. We have a long history of seeing people take up arms to prove their political point. From assassinations of presidents and other elected officials, to the murder of movement activists such as Martin Luther King Jr. to the violence committed against African-Americans in the Jim Crow South, to the white nationalists who blew up the federal government building in Oklahoma in 1995, we are a country that seen its share of these awful acts. There is good reason that many political observers keep warning President Trump and other elected officials not to play around with these ideas for electoral gain, for once they are unleashed, they become impossible to control.

However, the President has never taken this risk seriously. It is one thing to be extremely partisan but another to use dog whistles with violent organizations and to actually endorse the use of violence. The nation has seen different variations of Trump’s tendencies since he took office. Of course, one of the most defining moments of his presidency occurred after the violent marches in Charlottesville, Virginia, in August 2017, when he refused to come down hard on the neo-Nazis who entered the college town seeking to stir up racist and anti-Semitic sentiment.

After his barn-storming rallies or tweets where he continued to shock and awe by going well beyond the boundaries of legitimate presidential discourse, Trump brushed back critics who warned that in the wrong hands his blistering words could end up inciting real violence. We saw this recently when he made light of Montana’s Greg Gianforte having body slammed a journalist. “Any guy that can do a body slam, that’s my kind of guy,” the President said. Even in his tweet on Friday morning, he tweeted about the “Bomb stuff” that he suggested was distracting the public from the Republican rebound in the midterm elections.

Presidents have immense power. One of their greatest strengths is that their words have an impact on the national discourse unlike almost any other figure. Trump isn’t simply a reality television star anymore. When the President does something like retweet the image of himself knocking down a person with a CNN logo for a head, or when he says that members of his own cabinet are virtually guilty of “treason,” he sends out a dangerous message. While it is vital to remember that the President is not guilty of criminal actions committed in his name, the President is responsible for making statements that openly encourage this kind of behavior.

This week, the country learned just how that might play out on the ground in the hands of an unstable individual.

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It is likely that the President won’t take much of a break in his rhetoric. On Friday, he said that he thinks he’s been “toned down,” while warning, “I think I could really tone it up” given how “extremely unfair” the media has been to him. He will try to turn this to his advantage by suddenly becoming the champion of law and order while quickly focusing again on the refugees who he says are about to invade our borders. But really, the dangerous caravan was being driven by a violent person, brandishing Trump regalia, who seems to have been on a mission to Make America Great the only way that he knew how.