What contributed to the decline in union power and membership?
a.
a belief that “big labor” had become corrupt, fueled by republican rhetoric.

b.
automation and relocation of manufacturing plants.

c.
racism in the working class combined with a lack of political support for unions as democrats began to think that individual failure rather than systemic failure led to poverty.

d.
all of the above.

A - a belief that 'Big labor' had becom corrupt, fueled by Republican rhetoric.

Explanation:

In the beginning, labor unions fought for the rights of the workers (more pay, better/safer work conditions). Over time some of them have become corrupt, making more money for themselves than the workers - this led to a suspicion that all unions were corrupt, leading to a decline in membership and therefore power.