Final answer:
Johnson's rejection of the Fourteenth Amendment, his pro-Democratic speeches, and his refusal to cooperate with Congress united Radical, moderate, and conservative Republicans against him in 1866.
Step-by-step explanation:
In 1866, President Johnson's actions and decisions caused a rift between him and the Republicans in Congress, leading to a united front against him. Some of the key actions that contributed to this unity were Johnson's open rejection of the Fourteenth Amendment, his speeches during the 'Swing around the Circle' tour which had a pro-Democratic tone, and his unwillingness to compromise with Congress on Reconstruction. These actions and statements by Johnson pushed the Radical, moderate, and conservative Republicans to unite against him.