Finally, one moe from your site, Top contributotrs 1989 to now

Rank Organization Total ’89-’12 Dem % Repub % Tilt
1 ActBlue $57,470,970 99% 0%
2 AT&T Inc $48,025,567 44% 55%
3 American Fedn of State, County & Municipal Employees $46,382,548 94% 1%
4 National Assn of Realtors $41,403,426 47% 49%
5 Service Employees International Union $37,829,428 76% 2%
6 National Education Assn $37,197,739 82% 5%
7 Goldman Sachs $36,344,887 60% 39%
8 American Assn for Justice $35,200,054 88% 8%
9 Intl Brotherhood of Electrical Workers $34,637,147 97% 2%
10 Laborers Union $32,213,700 88% 7%

Five of the top ten are unions.
two are corporations.
two are quazi-unions /industry- NAR and the trial lawyers
#1 is a democrat PAC

11-20 is more interesting
7 are unions
1 is a corp
1 is a PAC, american medical association
1 is auto dealers is an industry PAC

Unions are the big spenders.

http://www.opensecrets.org/orgs/list.php?order=A