He also wrote a number of anti-slavery poems and his friendship with Newton, who was an avid anti-slavery campaigner, resulted in Cowper being asked to write in support of the Abolitionist campaign. Cowper wrote a poem called "The Negro's Complaint" (1788) which rapidly became very famous, and was often quoted by Martin Luther King Jr. during the 20th-century civil rights movement. He also wrote several other less well known poems on slavery in the 1780s, many of which attacked the idea that slavery was economically viable.