So, I've covered this topic before here, but I can provide a brief answer. Before the 1930s, party affiliation in the United States was not based primarily on ideology, but rather on geography and family tradition. This was mainly a legacy of the Civil War, where the Republicans were associated with the North, and the Democrats were associated with the South. There were exceptions to this rule, however, as seen by the dominance of the Democratic Tammany Hall political machine in New York. Family tradition was also part of the exception as seen by the branches of the Roosevelt family. Theodore Roosevelt came from the Oyster Bay branch of the family, which was traditionally Republican. However, his fifth-cousin Franklin Roosevelt came from the Hyde Park branch, which was traditionally Democratic. However, the general rule still stands. This was how progressives became prominent in both parties. It was not until the presidency of Franklin Roosevelt that ideology became the primary distinguishing characteristic of American political parties.