 He led Conservative attacks on the Mackenzie government throughout the 3rd Parliament.The Mackenzie government attempted to negotiate a new free trade agreement with the United States to replace the Canadian–American Reciprocity Treaty which the U.S. had abrogated in 1864.When Mackenzie proved unable to achieve reciprocity, Tupper began shifting toward protectionism and became a proponent of the National Policy which became a part of the Conservative platform in 1876.