Radical Republicans following the Civil War have been hailed as harbingers of the Civil Rights Movement which would usher in equal protection under the law for all Americans, including the freedmen of the South.
Yet many Radical Republicans were dedicated to Northern hegemony at the expense of the South. They wanted to further the growth of the American Government at the expense of the states and the people. Worst of all, Congressional firebrands like Pennsylvania's Congressman Thaddeus Stevens despised African Americans personally, catering to the constituency only to advance his reelection prospects and political dominance.
Case in point, the Radical Republicans made is a point to exclude repatriated Southern Legislators as much as possible. Rather than the 10% solution proffered by Abraham Lincoln, which would have permitted rebellious states to enter the Union when 10% of the population reaffirmed their allegiance to the Constitution of the United States, the Radicals imposed military law on the Southern States, with military commissions to supervise the proper treatment of blacks.
Sadly, these measures did not effect the necessary change to ensure proper treatment of recently released slaves. No government agency could have undone the steeping prejudices of the region which had imposed a race-based hierarchy one workers and owners. Furthermore, the arrangement impoverished the states at the expense of the Federal Government.
Only President Andrew Johnson, a pro-Union Democrat added to the Lincoln ticked to woo other pro-Union Democrats in the 1864 general election, stood in the way of the sweeping majorities in the House of Representatives and the United States Senate. He vetoed one bill after the other seeking to expand federal hegemony over the southern states, and by extension every state in the Union. He attacked the largess doled out by Congress, as well, yet every veto met with 2/3 of Congressmen overriding the veto.
Ultimately, the House brought up trumped-up charges against Johnson for violating the Office of Tenure Act, which forbade the executive from dismissing members of his cabinet with Congressional approval. Spying the flagrant unconstitutional power grab by Congress against the Presidency, Johnson summarily fired the Secretary of War.
Congress responded by passing articles of impeachment against the Senate, which then ushered in a trial in the Senate. Johnson avoided removal of office by one vote, a Republican from Kansas. Yet it was clear to all that Congress was very close to supreme dominance in Washington at the expense of President and Judiciary.
If nothing else, this whole episode in American political history points out the pitfalls of large majorities without the necessary checks and balances instituted by the Framers into the Constitution. President Johnson withstood the encroachments of Congress, yet he nearly lost his standing, and the Presidency its innate authority.