In 1836, Weld invited Rankin to join a group called "the Seventy", who were selected by the American Anti-Slavery Society to travel to churches throughout the Northern states preaching immediate emancipation ("immediatism") and forming local anti-slavery societies. Released by his congregation for one year to participate in the effort, Rankin's passion for the cause grew with the opposition to his "dangerous" views, even among many who opposed slavery but feared provoking a slave uprising. A bounty of up to $3,000 was placed on his life, and in 1841 he and his sons had to fight off attackers who came to burn his house and barn in the middle of the night.