Labor unions are legally recognized as representatives of workers in many industries in the United States. Their activity today centers on collective bargaining over wages, benefits, and working conditions for their membership, and on representing their members in disputes with management over violations of contract provisions. Larger unions also typically engage in lobbying activities and electioneering at the state and federal level.
As would come as no surprised, Liberals typically will support unions, and Conservatives hate them because they give the common middle class worker higher wages and more bargaining power in the face of companies with greater market power. More bargaining power, higher wages, and less desperation are three things the Conservative hates the most. The idea that a CEO would have to make less, and the company would make less profit while the workers make more makes the Conservatives skin crawl. If workers aren’t suffering and Conservatives cannot blame the victims for their suffering, Conservatives are miserable.
Resources for Union arguments.
Rewriting the Rules of the American Economy, Stifling Workers Voices, pg 70.