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.

Read More