The wave of Eastern European Jews, starting in the early 1880s, were generally more liberal or left-wing, and became the political majority.
Hasia Diner,
The Jews of the United States. 1654 to 2000 (2004), ch 5
Many of the latter came to America with experience in the
socialist,
anarchist, and
communist movements as well as the
Labor Bund, emanating from Eastern Europe. Many Jews rose to leadership positions in the early 20th century
American labor movement, and helped to found unions that played a major role in left-wing politics and, after 1936, in
Democratic Party politics.
[1] For most of the 20th century since 1936, the vast majority of Jews in the United States have been aligned with the Democratic Party.
Facts Matter