I'm not a liberal or a Democrat, but one way to read the chart is that the Great Depression was so horrifying that people turned to the Democrats and away from unfettered capitalism. That meant Keynesianism. Keynesianism required raising as well as lowering taxes as needed to manage the economy. By the late 1960's Republicans had become successful in making it politically all but impossible to raise taxes as needed. That led to the tremendous inflation of the 1970's, which Republicans/conservatives used to 'prove' the inefficacy of Keynesianism and to allow for a return, more and more, to unfettered capitalism. The result has been a return to a boom-then-bust economy.