This book tells the story of how abolitionist activists built the most transformative third-party movement in American history and set in motion changes that eventuated in the rise of the Republican Party, and ultimately, the Civil War and the abolition of American slavery. Because of the longstanding bifurcation between studies of the antislavery movement and studies of the sectional conflict, political abolitionists’ vital role in both has been too frequently overlooked. This book corrects this disconnect and shows how political abolitionists, working first through the Liberty Party and then the Free Soil Party, reshaped national politics. Savvy third-party leaders pioneered and disseminated the politically critical but often-misunderstood Slave Power concept, which this book reframes as an argument about party politics. Identifying the Second Party System of Whigs and Democrats as the mainstay of the Slave Power’s supremacy, political abolitionists insisted that only a party independent of slaveholder influence could overthrow the Slave Power’s control of the federal government. Through a series of shrewd electoral, lobbying, and legislative tactics, the Liberty and Free Soil Parties wielded power far beyond their numbers and helped reorient national political debate around slavery. Focusing especially on the U.S. Congress, political abolitionists popularized their Slave Power argument and helped generate controversy over slavery’s westward expansion to destroy the Second Party System and erect the Republican Party as the first major party independent of the Slave Power.