I’m a big believer that the one-two punch of MANUFACTURING + UNIONS produced such a large number of middle-income jobs,
And restriction on supply of workers.
Unions could not get traction, till the cut-off of cheap, readily replaceable scab labor
First, in the 1900-1920s with the restriction on immigration of first the Chinese, then most everywhere that wasn't Northern Europe
So when the Strikes started in the 1930s, there was no huge supply of people fresh off the Boat to replace the locked out workers, or the ones killed or driven off by the Goon Squad.
No more cheap replacements. The Companies had to start dealing with the workers
Lasted till the end of the restricted immigration.
Cesar Chavez noted the problem that illegal immigration caused his movement to get Farm Workers protectors.
Then add in Automation, the best way to replace workers. Machines are a multiplier. The more complex, the more effective
That's another reason why Wages decoupled from gains in Productivity