Final answer:
The Radical Republicans opposed President Johnson's Reconstruction plan because they perceived it as too lenient and insufficient in addressing the transformation of Southern society and ensuring rights for freed slaves.
Step-by-step explanation:
The Radical Republicans opposed President Andrew Johnson's Reconstruction plan because they viewed it as too lenient toward the South and ineffective in transforming Southern society. They favored a more stringent approach that would dismantle the old plantation system, provide greater protection and rights to freedmen, and ensure a complete reconstruction of Southern states. Whereas Johnson desired a swift and lenient reintegration of the South into the Union, the Radical Republicans sought to fundamentally reshape Southern society and governance through stronger federal control and civil rights provisions for freed slaves, especially after the Southern states' refusal to ratify the Fourteenth Amendment. Their disagreement with Johnson centered on the extent of federal power and the rights of African Americans post-war, leading to a clash that culminated in Congress taking control over Reconstruction.