Top 10 donors:
American Fedn of State, County & Municipal Employees $36,694,599
National Assn of Realtors $26,955,118
Assn of Trial Lawyers of America $25,300,041
National Education Assn $25,180,941
Intl Brotherhood of Electrical Workers $23,617,005
Service Employees International Union $23,354,475
Communications Workers of America $22,937,924
Laborers Union $22,812,207
Carpenters & Joiners Union $22,606,447
Teamsters Union $22,550,008
http://www.opensecrets.org/orgs/index.aspOn edit: There are many more corporations than there are unions. Therefore, any assertion that unions contribute more than corporations is absolutely falacious. Consider these statistics, also from Opensecrets.org:
THE BIG PICTURE
2002 CYCLEBusiness-Labor-Ideology Split in PAC, Soft & Individual Donations to Candidates and Parties
Grand Total Democrats Republicans Dem Repub
Business $1,008,406,673 $429,074,306 $577,020,735 43% 57%
Labor $96,584,777 $89,937,275 $6,465,902 93% 7%
Ideological $90,259,388 $49,010,162 $41,157,993 54% 46%
Other $108,024,969 $48,130,631 $59,271,482 45% 55%
Unknown $77,355,798 $21,589,286 $55,563,830 28% 72%
73% of 2002 contributions came from business, while only 7% came from unions.
http://www.opensecrets.org/bigpicture/blio.asp?cycle=2002