Complex sentence: In addition, he campaigned with the US Census Bureau to have the category of "mulatto" dropped in the 1930 and later censuses, thus depriving mixed-race people of recognition of their identity.
Split into simpler sentences: In addition, he campaigned with the US Census Bureau to have the category of "mulatto" dropped in the 1930 and later censuses. This deprived mixed-race people of recognition of their identity.