How did Radical Republicans win control of Congress?

The won 2/3 of the seats of the house of reps, and senate. they had power to override johnson vetoes and pass civil rights act to extend freedmen's bureau and they did.

Radical Republicans won over two-thirds of the seats in the House of Representatives and the Senate. They now had the power to override Johnson's vetoes and pass the Civil Rights Act and the bill to extend the Freedmen's Bureau, and they did so immediately. Congress had now taken charge of the South's reconstruction.