Complex sentence: The 1866 Congressional elections turned on the issue of Reconstruction, producing a sweeping Republican victory in the North, and providing the Radical Republicans with sufficient control of Congress to override Johnson's vetoes and commence their own "Radical Reconstruction" in 1867.
Split into simpler sentences: The 1866 Congressional elections turned on the issue of Reconstruction, and produced a sweeping Republican victory in the North. It gave the Radical Republicans enough control of Congress to override Johnson's vetoes and began what is called "Radical Reconstruction" in 1867.