The history of slavery and racial segregation in America makes clear the important role education and culture can play in fostering racism, and what happens when it becomes codified in law. And popular acceptance of the government's internment of Japanese-Americans as "enemy aliens" during World War II reminds us how quickly feelings toward an ethnic or racial group can change, especially at times of national emergency.
Trump's intolerance reminds us of something else: the power of a charismatic leader to exploit such sentiments for political gain.
The danger Trump poses lies with the kind of bond he has with his followers. It's an attachment based on submission to his authority, not to a party or principle. That's why he emphasizes the emotional content of his events -- feeling the love, or fending off "the haters"-- and grooms his fans' loyalty through an oath to his person.
Charismatic leaders have enormous influence over their followers, who look to them to set the tone for how they should think and behave. This becomes very dangerous when such leaders denigrate certain people, and then do nothing when their followers act on their words. Not only do they educate people to hate others, but they harness society's existing prejudices to rally supporters.
Trump has proceeded in textbook fashion. Early on, he identified suspect ethnic and racial groups (Muslims, Hispanics), declared others fair game (women, journalists, the disabled), and then shrugged when the aggression began.
Trump has used Twitter effectively to diffuse his racist messages throughout American society. On the heels of his racist tweets and retweets, Twitter has become a space where Trump's followers administer virtual public beatings to those who oppose their leader.
Trump's refusal to rein them in lets racists feel emboldened and even protected. Insults become the order of the day, in the street, on airplanes and the subway, and on social media, following a chain that originates with Trump's provocations.
His silence is especially notable given that the attacks have now extended to a group Trump professes to support: American Jews. Trump often cites his Jewish son-in-law and his daughter Ivanka's conversion to Judaism as evidence of his sympathies toward this religious minority.
The fact is that Trump's incitement of racism has caused an outbreak of anti-Semitism in America. It's most visible in the social media attacks
by Trump's followers on individuals of presumed Jewish name or heritage who criticize Trump in the media.
I am among those who have been subjected to an onslaught of racist slurs, from caricatures of me as an evil Jewess to veiled threats of rape to invitations to buy a one-way ticket to Israel. It was a surreal moment to be at an academic conference on the use of images by Fascist regimes the other day and discover that my Twitter feed had been invaded by images that reproduce or mimic National Socialist propaganda.
The most disturbing image was the simplest one: a puff of smoke in the sky. Me, incinerated, as were millions of Jews during World War II.
Since the start of his campaign, Trump has been testing the limits of what the American public and political class will accept as presidential behavior. He's been very direct about his intentions and targets, as charismatic authoritarians always are.
It's unfortunate that the GOP leadership has not wanted to listen. Romney is right to warn his party what may be store for all of us if Trump is elected.