Members of the Congress called Radical Republicans vowed to take control of Reconstruction.
Republicans were able to take control of Reconstruction because they had a solid majority - southern Democrats couldn't vote and northern Democrats had lost credibility (they were considered to be pro-Confederacy or, at least, in favor of settling the war vs. winning it). They did so out of profound philosophical differences with President Andrew Johnson over treatment of the former Confederate states and the newly-freed slaves.