It has continued to fall ever since
Does the ability to buy Toyotas hurt middle-class Americans? That is essentially the argument made by those who say falling union membership has harmed the middle class. But it holds little water. The decline of unions has hurt unions — while benefiting most other Americans.
Union membership certainly dropped sharply in the 1970s, the period when some argue things went south for the middle class. But the unions’ decline started well before that. Union density peaked during World War II and began falling significantly in the mid-1950s. Between 1954 and 1970, the proportion of workers belonging to unions fell by one-fourth. It has continued to fall ever since.
No one remembers the 1950s and 1960s as challenging for the middle class. Americans of all income levels prospered then, even as unions contracted. Union strength matters little for workers outside of unions.
This should not be surprising. Unions operate as legal labor cartels. They try to control the supply of labor in an industry so they can drive up its price — wages. Union members benefit, but those costs get passed on as higher prices. The price hikes make non-union consumers poorer. They also (unsurprisingly) reduce sales. Lower sales mean fewer jobs in that industry. Unions, like all cartels, benefit their members at the cost of greater losses to the rest of society.