Final answer:
Radical Republicans opposed President Andrew Johnson's lenient approach to Reconstruction because they aimed to protect freedmen's rights, redistribute land, and eradicate the old Southern aristocracy, seeking a radical transformation of Southern society.
Step-by-step explanation:
The Radical Republicans in Congress opposed Presidential Reconstruction under President Andrew Johnson because they sought a more thorough transformation of Southern society, which included not just the abolition of slavery but also the protection of the freedmen's rights, the redistribution of land, and the restructuring of Southern politics and economics to eradicate the old aristocracy. Johnson's lenient approach to Reconstruction, aimed at a swift reunification without significant changes to the South's structure, was at odds with the Radical Republicans' vision of extensively overhauling the South. Their conflict resulted in the Radicals gaining control of Congress and passing legislation that initiated a more radical Reconstruction, over Johnson's vetoes.
Radical Republicans like Thaddeus Stevens and Charles Sumner wanted to address the issues of racial equality and ensure a power shift in the South from the planter class to the freedmen. They advocated for measures such as integrating schools, enfranchising Black men while disenfranchising former Confederates, and disrupting the plantation system through land redistribution. The Southern refusal to ratify the Fourteenth Amendment and the imposition of Black Codes confirmed the fears of Republicans that Johnson's approach was insufficient and drove them to seize the initiative in shaping Reconstruction policy, leading to a series of legislative actions that undercut Johnson's Presidential Reconstruction efforts.