Under Roosevelt, the modern dynamic between the mainstream parties and socialism was crystallized: The Democrats would borrow from socialist policy proposals while disavowing the ideology in attempts to weather rhetorical attacks from Republicans. When he was slammed as a socialist by Republican politicians, Roosevelt defended himself by denying the association, rather than defending the merits of socialist ideas. When President Harry S. Truman was accused of being a socialist, he responded by diminishing “socialist” to a throwaway pejorative, saying in 1950, “Confronted by the great record of this country, and the tremendous promise of its future, all they can croak is ‘Socialism!’ ”

Socialism became truly toxic in the mid-20th century, building on the decades that came before. McCarthy’s decade-long harassment of those he accused of being socialists or communists began in 1947. For the rest of the century, “socialist” was one of the dirtiest words in politics, helped along by Cold Warriors such as President Ronald Reagan. The rise of the counterculture in the 1960s and 1970s didn’t provide a foothold for socialists like the upheaval of the late 1800s and early 1900s had; Bernie Sanders is the only socialist activist to emerge from that period with influence.