The history of assimilation can be looked at several ways, all of them revealing.
1. Perhaps the single most important point is that mass immigration was ended around WWI. The restrictive legislation of 1917 (the literacy act), 1921, and 1924 (Johnson-Reed) ended the great wave of mass immigration. Immigration continued on a much smaller scale and the composition was shifted. Because the new laws were based on national quotas, some countries were de-facto unrestricted (the UK, Ireland, etc.) while other reached their annual quotas rather quickly.
The immigration restrictions had several positive effects. Wages and working conditions improved for immigrants over time. Since most immigrants worked in manufacturing, wage gains in manufacturing were particularly positive. Sectoral differentiation (by accident) favored immigrants in this period. Manufacturing was booming and farming was in decline. Since the immigrant population was predominantly urban and employed in goods production, this was a plus.
Living conditions for immigrants clearly improved after mass immigration ended. The horrific tenements slums of the 1900 period were largely empty by the 1930s. Without new massive waves of immigrants, the prior cohorts were able to move up the social ladder into better housing (among other things).
The immigration restrictions of the WWI period clearly aided assimilation in practical ways (waves and living conditions). However, they also sent a very important message to the immigrant communities. America rejected ‘diversity’ and demanded that the immigrants embrace America rather than the other way around. There points were were well understood back then and it was widely understood that the immigration cutoff (substantial reduction actually) had accelerated assimilation.
2. The America of the 1920s and later was vastly better suited to assimilating immigrants than our nation today. We had a booming job market, no welfare state, middle-class unions (starting in the 1930s), English imposition, disciplined education, no multiculturalism, no bilingualism, no victimization ideology, intact families, rigorous law enforcement, etc. Beyond that, ‘Americanization’ (assimilation) was a widely embraced ideal and promoted heavily. Now we have the pernicious and very dominant ideology of ‘diversity’.
3. In spite of much more favorable circumstances, assimilation took time. Some groups assimilated much faster than others, but three generations were typically enough to achieve earnings party with old stock natives. The mythology is that one generation was sufficient. It wasn’t. Even well after WWII, ethnic differences in earnings, social status, etc. were measurable.
The political assimilation of ‘Great Wave’ immigrants was relatively slow but did occur. In this context, I will use the Catholic vote as a proxy for ‘Great Wave’ immigrants. By some measures, 1928 marks the zenith of Catholic alienation from the political mainstream. Al Smith got 90% of the Catholic vote (apparently) and was still easily defeated by Hoover. Indeed, he failed to carry his home state of New York (he was a former governor or New York). By the time JFK was elected, Catholic support for the Democratic party had fallen markedly. He won the Catholic vote, but by a notably smaller margin than Al Smith. A poll of Fordham University students in 1960 showed that most of the Catholic students favored Nixon over Kennedy (the Jewish students at Fordham favored Kennedy). Students at other Catholic schools favored Nixon as well.
Perhaps more relevantly, Eisenhower captured a majority of the Catholic vote in 1956. In subsequent presidential elections, Republicans were able to easily capture the Catholic vote (if they could win at all).
It’s worth noting that in an earlier era, ethnics were polled separately and political differences by ethnicity were material. By the 1980s, this practice had essentially disappeared because ethnic voting patterns were no longer different enough to measure.
However, voting patterns are not the most important aspect of assimilation in my opinion. The assimilation of American values is far more important. Once again, studies show that ‘Great Wave’ immigrants embraced the values of old-stock natives (personal responsibility, individual effort, hard work and education as the keys to advancement, national loyalty, limited government, etc.). Basically, ‘Anglo-conformity’ worked.
It is wrong to suggest that Jewish Americans don’t like WASP America. More like they resent it. The American Jewish community remains (privately) obsessed with the efforts of WASPs to exclude Jews from elite society. The fact that many of these efforts were more than 100 years ago doesn’t appear to matter. Nor does the fact that even with Jewish quotas in place, Jews were vastly overrepresented at Harvard and other elite schools.
The bigger picture, that America has been a wonderfully hospitable nation with immense opportunities for personal and professional advancement is subordinated to resentment of country club prejudice in the 1920s. As a consequence, American Jews define themselves as outsiders and vote accordingly.
As these notes should indicate, America was once a much better place for immigrants and their families. The historic advantages of assimilation, ‘Americanization’, Anglo-conformity, immigration restrictions, and a strong economy are all gone (along with quite a few other historic virtues). It’s also true that the immigrants were better historically. They were much more likely to be skilled, educated, etc. The ‘Great Wave’ immigrants has much lower skill levels and the turn of American society against immigration was largely a consequence. Contemporary mythology emphasizes the role of nativism in the restrictions of the 1920s. Declining skill levels provides a different and more germane explanation.