The top 20 political donors over the past twenty years. I believe that is a dynamic list, but as of today, fourteen of the top twenty donors are heavily Democratic. Four donors are fence-sitters, and two (at positions 18 and 20), lean Republican.

Here's the breakdown, with the percent of their donations that is sent to Democrats in parenthesis:

1. ActBlue (99%)
   
2. AT&T Inc (44%)
   
3. AFSCME (98%)
   
4. Nat'l Assn of Realtors (49%)
   
5. Goldman Sachs (61%)
   
6. American Assn for Justice (90%)
   
7. Int'l Brotherhood of Electrical Workers (97%)
   
8. National Education Assn (93%)
   
9. Laborers Union (92%)
   
10. Teamsters Union (93%)
    
11. Carpenters & Joiners Union (89%)
    
12. SEIU (95%)
    
13. American Federation of Teachers (98%)
    
14. Communications Workers of America (98%)
    
15. Citigroup Inc (50%)
    
16. American Medical Assn (40%)
    
17. United Auto Workers (98%)
    
18. National Audo Dealers Assn (32%)
    
19. Machinists & Aerospace Workers Union (98%)
    
20. United Parcel Service (36%)