Most of the changes under discussion occurred between 1932-1972, a time when liberal democrats dominated Congress and pushed through many government programs and policies. Often, even so-called "right wingers" like Nixon were more centrist, adopting many "liberal" causes and methods such as wage and price controls, instituting the EPA, etc,....
The few conservative voices, like Governor Reagan, were viewed as an anamoly, or they were connected to race politics, as Wallace was when won a few states on a third party platform. The Left was actually beyond "liberal" at some points (Weathermen in late 60s and 70s, Socialists/Communists in the 30s and 40s).