There are a number of trends that repeated at least once during the 20th century. Let us look at two of them.
First, there is the trend of increasing immigration and the nativism that comes with such increases. The first time that this happened was in the early part of the century. There was a great deal of “new immigration” by people from Southern and Eastern Europe before World War I. This bothered many people because the new immigrants were seen as too different from “native” Americans. The nativism led to immigration restriction in the 1920s. This trend repeated at the end of the century. This time, the immigration was from Latin America. It led to the sort of anti-immigrant attitudes that have remained prevalent to the present. There was one major difference in that part of the issue in the later wave of immigration was the fact that the immigration was illegal. Even so, both of these were instances of nativism arising from a wave of immigration.
Second, there was a trend of changing social values that led to backlashes. The first of these was in the 1920s. The new values of that era were typified by the “flappers” and other aspects of the “Jazz Age.” These new ways led to backlashes in the form of such things as religious fundamentalism. This trend repeated in the 1960s with the counterculture. The hippies and others went against traditional values, sparking a backlash. This backlash took the form of Nixon’s “Silent Majority.” The conflict between the traditional values and new ways can still be seen playing out to this day.