So-called Radical Republicans drove and passed the 13th (1865), 14th (1868) and 15th (1870) Amendments, and tried to impeach Eastern Tennessee Democrat President Andrew Johnson (1868) because, basically, he was thought way too lenient on former Confederates.
Curious that most of our radical pro-Confederates these days consider Johnson just another Northern devil.
Many people say that somehow Lincoln would have been more forgiving than Johnson, had he lived.
Regardless, that period from Appomattox in April 1865 until the presidential election of 1876 is pretty murky and misunderstood.
Certainly I'm no expert on it, and am disinclined to defend one particular group or another.
Radical Republicans tried to impeach Johnson and had considered running someone against Lincoln in 1864. Their own plans for Reconstruction resembled something that the Bolsheviks would try later. An extremely vengeful crowd who would have tried and executed Davis, Lee, and whole passel of Confederates. They lost much of their power when Grant took office. Grant and Sherman and most the Union command had no use for them.