The Radical Republicans in Congress were angered by Johnson's actions. They refused to allow Southern representatives and senators to take their seats in Congress. In 1866, the Congress passed the Civil Rights Bill, which granted African Americans equal protection under the law with whites.
Johnson's refusal to support the legislation that had united the Republican majority in Congress proved fatal to his program, for it had driven the moderates into the Radical camp. Johnson now lacked the support he needed to get his own policies approved and enacted.
Two Reasons: 1)They considered the firing of Stanton by Johnson to be a crime under The Tenure of Office Act. 2)It was a Political Move to disenfranchise Johnson after his Alienation of Congress at their attempts at reconstruction.
Following the end of the Civil War, the Radical Republicans took a stance against Lincoln's successor, President Andrew Johnson. They saw no effort in this new President's policies to create rights for Blacks.