Why does Senator Soule from Louisiana take a strong interest in Northup's case and indicate that "it was the duty and interest of every planter in his state to aid in restoring me to freedom"

There were several prominent politicians that took an interest in Northup's case. There was the governor of New York, the United States Secretary of War, and Senator Soule from Louisiana. Northup’s story had gathered some traction and it was possible that they had personal political motives for helping Northup. I don't recall a specific motive for Soule.