What was significant about the results of the 1866 congressional elections?

It was seen as a referendum against the policies of President Johnson. During this time, he even lost the support of his own party for many of the policies he had advocated for during Reconstruction. When the Radical Republicans took over the Congress during this electoral period, it led to their being able to set the terms of Reconstruction instead of the President.

That president would be none other than president warren g harding , return to say of life before wwi