From Eugenics and Economics in the Progressive Era by Thomas C. Leonard (link via Don Boudreaux, I think).
It was a scholarly fashion, circa 1890, to declare the U.S. frontier “closed” and to sound a Malthusian alarm about excess American population growth. But the professional economists who wrote on immigration increasingly emphasized not the quantity of immigrants, but their quality. “If we could leave out of account the question of race and eugenics,” Irving Fisher (1921, pp. 226–227) said in his presidential address to the Eugenics Research Association, “I should, as an economist, be inclined to the view that unrestricted immigration . . . is economically advantageous to the country as a whole . . . .” But, cautioned Fisher, “the core of the problem of immigration is . . . one of race and eugenics,” the problem of the Anglo-Saxon racial stock being overwhelmed by racially inferior “defectives, delinquents and dependents.”
Fear and dislike of immigrants certainly were not new in the Progressive Era. But leading professional economists were among the first to provide scientific respectability for immigration restriction on racial grounds.2 They justified racebased immigration restriction as a remedy for “race suicide,” a Progressive Era term for the process by which racially superior stock (“natives”) is outbred by a more prolific, but racially inferior stock (immigrants).
Note that the authors of the time were not using race as we do -- by "other races" whose immigration into the US was going to destroy us, they meant Southern Italy, Russia, Austria, Hungary, and the rest of Eastern Europe. Fifty years earlier, they would have meant the Irish. All of who we would today consider part of the backbone of America. Why do we have to take these ideas seriously today when they have been wrong so consistently in the past?