After 1932, it seems that the Democrats and Republicans more or less coalesced into their modern forms. However, before 1932 it seems that each party had conservative, moderate, and progressive factions. What were the issues that united all the Democrats into one party and all the Republicans into another?
For example, why did some progressives (Fiorello La Guardia, Robert La Follette, Jeannette Rankin, Richard Pettigrew, George Norris) join the Republican Party, while other progressives (Glen Taylor, FDR, Rex Tugwell, William Jennings Bryan) joined the Democrats?
It might help if you give a narrower timeframe as the Democratic Party became prominent in the 1830s and the Republican Party in the 1850s.