He supported an argument called common sense. He actually wrote it himself.
Both were born into slavery, and escaped into slavery. While Tubman physically guided slaves along the route to freedom, Douglass wrote and spoke to white audiences about the travails of living first as a slave and then as a black man, subject to the racism of the time.
New diseases caused many deaths, european colonist began using enslaved laborers, spain used the encomienda system in the americas, native people were forced from their land.