By 1848 the United States had seized vast new western territories from Mexico, leading Sumner and his faction to join with the Liberty party and northern antislavery Democrats to create the new Free Soil party. In so doing Sumner never hesitated in attacking former friends, whom he said supported the slave power in an alliance between "the lords of the lash and the lords of the loom." Such attacks were fast becoming a Sumner trademark, as he spared no one who opposed his goals. The young reformer did not confine his concern for racial justice to territorial slavery. In 1849 he argued in court for the integration of Boston's public schools and, while losing his case, presented arguments for social change far in advance of his times.
Participation in the Free Soil movement gave Sumner his first taste of political prominence, which he quickly utilized to secure public office. In a skillful political maneuver in 1851, Massachusetts Free Soilers formed a coalition in the state legislature with Democrats and secured Sumner's election to the U.S. Senate. Thus began his long tenure as an outspoken reformer in Congress.