This dissertation reintegrates abolitionism into the main currents of U.S. political history. Because of a bifurcation between studies of the American antislavery movement and political histories of the sectional conflict, modern scholars have drastically underestimated the significance of abolitionist political activism. Historians often characterize political abolitionists as naïve idealists or separatist moral purists, but I recast them as practical, effective politicians, who capitalized on rare openings in American political institutions to achieve outsized influence in the face of a robust two-party system. Third-party abolitionists shaped national debate far beyond their numbers and played central roles in the emergence of the Republican Party.

Over the second half of the 1830s, political abolitionists devised the Slave Power concept, claiming that slaveholder control of the federal government endangered American democracy; this would later become the Republicans' most important appeal. Integrating this argument with an institutional analysis of the Second Party System, antislavery activists assailed the Whigs and Democrats--cross-sectional parties that incorporated antislavery voices while supporting proslavery policies--as beholden to the Slave Power. This analysis thus provided the rationale for creation of the abolitionist Liberty Party and then became its chief rhetorical tool.

Liberty partisans cast all elections as contests against the Slave Power and repeatedly forced slavery into political debate by controlling balances of power in many northern locales. Meanwhile, they developed a sophisticated lobbying strategy to exploit Congress as a public forum that could be made to magnify and widely disseminate the Slave Power argument. As northern Whigs and Democrats faced new antislavery electoral pressures and chafed under the Slave Power's increasing exactions, Liberty leaders redoubled their efforts to pry antislavery dissidents from the major parties. In the process Liberty men paved the way for a broader anti-Slave Power coalition and helped found the Free Soil Party in 1848. A small but dedicated Free Soil congressional bloc then built on Liberty tactics to further harness congressional debate as a platform for dramatizing the Slave Power's control of national policymaking. When the 1854 Kansas-Nebraska Act roused northern voters and deranged once-stable party allegiances, Free Soil leaders in and out of Congress seized on the opportunity to spearhead a party uniting all opponents of the Slave Power. In helping propel this new Republican Party to northern preeminence by 1856, erstwhile Liberty men and Free Soilers finally foresaw the end of the Slave Power's national supremacy, and, ultimately, of slavery itself.