JMA-PSOS

This is a very good pro-union union-funded article from the EPI. Part of its summary/conclusion (supported with lots of data and references):

High union density is consistently associated with a much broader set of positive spillover effects across multiple dimensions. Not only do unions directly benefit the workers they represent, but their political advocacy helps to drive an array of strongly positive outcomes more broadly, especially in states where unions represent a sizeable share of the workforce. These positive outcomes include wage increases, better health benefits, easier access to unemployment insurance, access to paid sick leave, access to paid family and medical leave, and unrestricted voting opportunities.

For a history of unions (e.g. decrease of unionization in USA from 30% in the 50s and 60s to 10% today), see Trade Union Economics or Labor unions in the United States.