The Securities and Exchange Commission (SEC) plans to propose rules that may diminish the longstanding importance of credit ratings across various markets, including the $3.4 trillion money-market industry, in the latest blow to the rating business stemming from the credit crunch, the Wall Street Journal reported today. The most significant portion of the rules, to be proposed tomorrow, would make it possible for U.S. money-market funds to invest in short-term debt without regard to ratings put on those securities by firms such as Moody’s Investors Service and Standard & Poor’s. Currently, SEC rules generally require that money-market funds purchase only short-term debt with high investment-grade ratings. The new rule would put more discretion in the hands of money managers to determine whether the debt is investment grade.