I took a class on the Radical Republicans (Reconstruction, actually, but they played a big part in it).  Simply put, the South thought of the Radical Republicans as occupants.  They took over the Confederate States and ran them as territories: military governors and strict laws.  The former slaves considered them saviors.  They instituted laws directed at helping out their freedom, to a point.  There were a lot of empty promises made by these Republicans, but it is no surprise that almost all former slaves registered as Repulbicans.  The north is where most of the Republicans came from.  For more information on the northern view, look for anything regarding Andrew Johnson's tenure in office.  For the former slaves and southern perspectives, look for information on Reconstruction or the Hayes' election, which ended Reconstruction.