Story highlights Ruth Ben-Ghiat: Mitt Romney accused Donald Trump of creating 'trickle-down' racism. She says history offers many examples

She says Trump's bigotry emboldens racists. She was victim of a surge of anti-Semitic slurs on Twitter

Ruth Ben-Ghiat is a professor of history and Italian studies at New York University. Her latest book is "Italian Fascism's Empire Cinema." The opinions expressed in this commentary are solely those of the author.

(CNN) How do people become racists? How is bigotry spread and sustained in society? Mitt Romney's comments that Donald Trump's election could legitimize "trickle-down" racism in America make us think twice about these questions.

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.

Ruth Ben-Ghiat

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.

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