The Vermont senator proposed that the agencies — which include Standard & Poor's, Moody's and Fitch — should not depend on Wall Street clients. Some industry watchers have complained that the client-customer relationship between those firms and financial institutions could create a conflict of interest.
Sanders also said that he would seek to reform the Fed so it stops paying financial institutions interest for money kept with the central bank. He said that the Fed should charge those firms a fee for keeping reserves.
The presidential candidate also said that if he were to win the race, banking industry executives would be banned from serving on the board of the Fed.
Sanders, a democratic socialist, is challenging front-runner Hillary Clinton, a former secretary of state and U.S. senator from New York, and former Maryland Gov. Martin O'Malley for the Democratic nomination.
— CNBC's Everett Rosenfeld contributed to this report.