In Virginia prior to 1920, for example, a person was legally white if having seven-eights or more white ancestry. The one-drop rule originated in some Southern United States in the late 19th century, likely in response to whites' attempt to maintain white supremacy and limit black political power following the Democrats' regaining control of state legislatures in the late 1870s. The first year in which the U.S. Census dropped the mulatto category was 1920; that year enumerators were instructed to classify people in a binary way as white or black. This was a result of the Southern-dominated Congress convincing the Census Bureau to change its rules.