OpenSecrets.org
The Center for Responsive Politics (CRP) is a non-profit, nonpartisan research group based in Washington, D.C. that tracks the effects of money and lobbying on elections and public policy. It maintains a public online database of its information.[1]
Their database OpenSecrets.org[2] allows users to track federal campaign contributions and lobbying, by lobbying firms, individual lobbyists, industry, federal agency, and bills. Other resources include the personal financial disclosures of all members of the U.S. Congress, the president, and top members of the administration. Users can also search by ZIP codes to learn how their neighbors are allocating their political contributions.
Source: Wikipedia: http://en.wikipedia.org/wiki/Center_for_Responsive_Politics
Totals include reported contributions from PACs and individuals affiliated with Heavy Hitter organizations, which are defined as the top overall donors to candidates, parties, Leadership PACs and other committees. Contributions to outside groups like super PACs do not factor into an organization's designation as a Heavy Hitter, however the totals below do include contributions by Heavy Hitters to such groups, as well as contributions to candidates, parties, Leadership PACs and other committees. Furthermore, the totals do not include contributions to politically active nonprofits, which are not disclosed to the public.