It's public information if you want to look it up. Go ahead. Search for the donor records of just about any Democrat candidate for any significant office. What you'll find is that the list is heavily populated with labor unions, many of them representing public employees.
This has long been considered a racket, and for good reason. Democrats pass laws that compel workers to join unions and pay them dues whether the workers want to or not, and then the unions turn around and contribute substantial amounts of the money collected back to Democrat candidates to support their campaigns, even though some percentage of the workers paying the dues may not support the Democrat candidates and don't want their money going to support them.
