- Carnegie Endowment for International Peace (CEIP)
- Center for Defense Information (CDI)
- Center for Responsive Politics (CRP)
- Center for Strategic and International Studies (CSIS)
- Center for Study of Responsive Law
- Children's Defense Fund (CDF)
- Choice in Dying (CID)
- Christian Coalition (CC)
- Citizens Committee for the Right to Keep and Bear Arms (CCRKBA)
- Citizens for Tax Justice (CTJ)
- Co-Op America
- Common Cause (CC)
- Communications Workers of America (CWA)
- Concerned Women for America (CWA)
- Congressional Black Caucus (CBC)
- Consumer Alert
- Consumer Federation of America (CFA)
- Consumers Union of the United States (CU)
- Council of Better Business Bureaus (CBBB)
- Council on Competitiveness
- Council on Foreign Relations (CFR)
- Credit Union National Association and Affiliates (CUNA)
