Donald Trump knows the United States will never deport eleven million undocumented immigrants or do away with birthright citizenship. But what if we did—what would be the political impact if Trump and other angry nativists in the GOP actually achieved most or all the changes they desire, cutting immigration back sharply?

We already know, because something very similar happened once before in American history. Ninety years ago, two Republican presidents—Warren Harding and Calvin Coolidge—and a Congress dominated by Republicans enacted equally harsh policies against immigrants. Their success helped usher in the longest period of one-party rule in the 20th century. But it was the Democrats, not the GOP, who benefited, in one of the most whopping instances of unintentional consequences in American political history.

During the 1920s, federal lawmakers reversed the traditional policy of welcoming newcomers from nearly every land. Fear of foreigners carrying the bacillus of Bolshevism from Europe and of diluting the purity of the “Nordic” race led them to pass the most sweeping restrictions in U.S. history. By large majorities, Congress enacted quotas that explicitly discriminated against would-be immigrants from southern and eastern Europe and banned all Arabs and all Asians except for Filipinos, who were then U.S. colonial subjects. In supporting the restrictive Johnson-Reed Act of 1924, one senator proudly exclaimed, “Thank God we have in America perhaps the largest percentage of any country in the world of the pure, unadulterated Anglo-Saxon stock … We now have sufficient population in our country for us to shut the door and to breed up a pure, unadulterated American citizenship.”

The new policies were effective: Over 18 million people migrated to the U.S. between 1880 and 1920. From 1930 to 1960, during the new era of highly restricted immigration, only four million made the trip.

But the political backlash from that dramatic shift in demographics was fierce. Immigrants from places like Poland, Italy, and Russia who already lived in the U.S. and their American-born children deeply resented quotas that barred them from bringing over their relatives and friends. Most also despised the prohibition of alcohol, which they viewed as an attack by evangelical Protestants on their cultures and their right to imbibe any beverage they chose. ...