How could you get the US to have a long period of a free market economic consensus from the depression era and then start moving to the left in the 70s and fully embrace Keynsianism starting in 1980?
Somehow the Great Depression and World War 2 (OTL the main events responsible for the post-war consensus) and be able to prolong the surviving free-market consensus until there's a major (world) war around say 1975.