So Warren Harding is the only Republican to win even a plurality of the Jewish vote? What caused Republican support to drop off so heavily and so suddenly in 1924 and 1928?
Harding was the last GOP candidate to win the Jewish vote. That doesn't mean he was the only one to do so. The Jewish vote switched so hevily in 1924 because that is when Democratic political machines started getting very strong in the large cities. The 1920s is also when the Democrats become perceived as the party of urban interests, and most Jews back then (and now) lived in urban areas.
The Democratic Party had strong urban machines long before then, especially among immigrants.
