10. Service Employees International Union
Total Contribution: $222,520,804
Founded in 1921, the Service Employees International Union represents over 1.9 million workers across the United States. The organization believes in the dignity and worth of such workers and fights for the protection of their rights. Known for endorsing most Democratic candidates, the SEIU has donated over $222 million in total contributions. Hillary Clinton was able to secure their influential endorsement in November, 2015.