Unions were also a source of funding for the Democratic party. As it lost funding from unions, the party became increasingly dependent on the wealthy and corporate benefactors. This caused it to side with policies that further hurt blue collar workers. Wither the New Deal coalition.

https://en.wikipedia.org/wiki/History_of_the_United_States_D...

https://www.theatlantic.com/magazine/archive/2017/07/whats-w...