To look up contributions to U.S. Federal political campaigns, visit Open Secrets and/or the sites posted by the
Federal Election Commission (FEC), the House of Representatives or the Senate. Federal data is also available on Lexis (CMPGN) and CQ's First Street. Legistorm posts a database of privately financed travel.
In addition, political parties and other political organizations raising over $25,000 are required to register with the IRS using Form 8871, and those raising over $50,000 are required to list their contributors and the expenditures on Form 8872. Some must also file Form 990 tax returns. You can look up these forms through the IRS's Political Organization Disclosures database.
For state campaigns, visit Open Secrets, FollowTheMoney.org and/or any relevant state Web site (e.g., the California Voter Foundation or the Maryland Campaign Reporting System).
Westlaw has a database of contributors and contributions made at the state and local level going back to 1996 (POLITICAL-DONORS).
FEC Advisory Opinions: The Federal Election Commission posts its Advisory Opinions back to 1977. You can also search for FEC Advisory Opinions on Westlaw.