When Congress took control of Reconstruction in early 1867, Spencer returned to Alabama and secured an appointment as a register in bankruptcy. His wife joined him in July, and in addition to working the bankruptcy circuit, he labored to "carry Alabama and secure it permanently to the Republican party."… He campaigned aggressively for Republican candidates in the state elections of February 1868, and the new legislature rewarded him with a U.S. Senate seat. Reelected four years later, Spencer served from Alabama's readmission to the Union in July 1868 until 1879--well after conservative Democrats had "redeemed" the state from Republican rule in 1874. It was a turbulent tenure--conservatives immediately tagged him with the opprobrious epithet of "carpetbagger," and Spencer and other Republicans faced constant ostracism and intimidation from an overwhelming majority of white southerners.
A pivotal figure in Alabama politics during the 1870s, Spencer quickly emerged as a shrewd political infighter who used power and patronage to reward friends and punish enemies--both inside and outside his party. In Congress, Spencer acquired a reputation as a stalwart politician, a regular Grant Republican, yet his rhetoric and activity also reflected an idealistic concern for the welfare of the freedmen and a missionary belief in the necessity of southern "regeneration"--reshaping the region into a more progressive society. On economic measures, he energetically served constituent interests, especially as a member of the Senate Commerce Committee, where he steered federal funds to the South through the annual rivers and harbors appropriations.