Answer
Wendell Phillips, a prominent abolitionist and orator, referred to Harriet Martineau as the "greatest American abolitionist." Martineau, an English writer and social reformer, played a significant role in shaping public opinion against slavery in both the United States and Great Britain. Her insightful observations and writings, based on her extensive travels in the antebellum South, challenged the institution of slavery and provided valuable ammunition to the abolitionist movement.