Under the Constitution, Congress is “the judge of the Elections, Returns and Qualifications of its own Members” (Article 1, Section 5). Using this power, Republican majorities in both houses had refused to admit southern delegations when Congress convened in December 1865, effectively blocking Johnson’s program. Hoping to mollify Congress, some southern states dropped the most objectionable provisions from their Black Codes. But at the same time, antiblack violence erupted in various parts of the South.

Congressional Republicans concluded that the federal government had to intervene. Back in March 1865, Congress had established the Freedmen’s Bureau to aid displaced blacks and other war refugees. In early 1866, Congress voted to extend the bureau, gave it direct funding for the first time, and authorized its agents to investigate southern abuses. Even more extraordinary was the Civil Rights Act of 1866, which declared formerly enslaved people to be citizens and granted them equal protection and rights of contract, with full access to the courts.

These bills provoked bitter conflict with Johnson, who vetoed them both. Johnson’s racism, hitherto publicly muted, now blazed forth: “This is a country for white men, and by God, as long as I am president, it shall be a government for white men.” Galvanized, Republicans in Congress gathered two-thirds majorities and overrode both vetoes, passing the Civil Rights Act in April 1866 and the Freedmen’s Bureau law four months later. Their resolve was reinforced by continued upheaval in the South. In addition to the violence in Memphis, twenty-four black political leaders and their allies in Arkansas were murdered and their homes burned.

Anxious to protect freedpeople and reassert Republican power in the South, Congress took further measures to sustain civil rights. In what became the Fourteenth Amendment (1868), it declared that “all persons born or naturalized in the United States” were citizens. No state could abridge “the privileges or immunities of citizens of the United States”; deprive “any person of life, liberty, or property, without due process of law”; or deny anyone “equal protection.” In a stunning increase of federal power, the Fourteenth Amendment declared that when people’s essential rights were at stake, national citizenship henceforth took priority over citizenship in a state.

Johnson opposed ratification, but public opinion had swung against him. In the 1866 congressional elections, voters gave Republicans a 3-to-1 majority in Congress. Power shifted to the so-called Radical Republicans, who sought sweeping transformations in the defeated South. Radicals’ leader in the Senate was Charles Sumner of Massachusetts, the fiery abolitionist who in 1856 had been nearly beaten to death by South Carolina congressman Preston Brooks. Radicals in the House followed Thaddeus Stevens of Pennsylvania, a passionate advocate of freedmen’s political and economic rights. With such men at the fore, and with congressional Republicans now numerous and united enough to override Johnson’s vetoes on many questions, Congress proceeded to remake Reconstruction.