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I often open my articles on Trumpism with explorations of American history. I’ve spent much of the past four years trying to illuminate the historical and ideological antecedents to Donald Trump, to show how America got to this point.

So I want to be very clear about what the country saw last night, as an American president incited a chant of “Send her back!” aimed at a Somali-born member of Congress: America has not been here before.

White nationalism was a formal or informal governing doctrine of the United States until 1965, or for most of its existence as a country. Racist demagogues, from Andrew Johnson to Woodrow Wilson, have occupied the White House. Trump has predecessors, such as Calvin Coolidge, who imposed racist immigration restrictions designed to preserve a white demographic majority. Prior presidents, such as Richard Nixon, have exploited racial division for political gain. But we have never seen an American president make a U.S. representative, a refugee, an American citizen, a woman of color, and a religious minority an object of hate for the political masses, in a deliberate attempt to turn the country against his fellow Americans who share any of those traits. Trump is assailing the moral foundations of the multiracial democracy Americans have struggled to bring into existence since 1965, and unless Trumpism is defeated, that fragile project will fail.

Nevertheless, most of Trump’s predecessors had something he does not yet have: the support of a majority of the electorate. Ilhan Omar’s prominence as a Republican target comes not, as conservatives might argue, simply because her policy views are left-wing. Neither is it because, as some liberals have supposed, she is an unmatched political talent. She has emerged as an Emmanuel Goldstein for the Trumpist right because as a black woman, a Muslim, an immigrant, and a progressive member of Congress, she represents in vivid terms a threat to the nation Trumpists fear they are losing.

To attack Omar is to attack a symbol of the demographic change that is eroding white cultural and political hegemony, the defense of which is Trumpism’s only sincere political purpose. Many of the president’s most outrageous comments have been delivered extemporaneously, when he departs from his prepared remarks. Last night, though, his attacks on Omar were carefully scripted, written out by his staff and then read off a teleprompter. To defend the remarks as politically shrewd is to confess that the president is deliberately campaigning on the claim that only white people can truly, irrevocably be American.