April 28 (Bloomberg) — The biggest money managers are asking U.S. Congress to fix a largely frozen part of the mortgage-bond market with a step that would echo moves taken during the Great Depression.

The Association of Institutional Investors, whose members include Pacific Investment Management Co., Fidelity Investments and Schroders Plc, asked lawmakers in a letter today to create an "unambiguous fiduciary standard" for trustees of mortgage bonds without government backing as a Senate committee considers a bill that would overhaul the $9.4 trillion home-loan market by replacing U.S.-controlled Fannie Mae and Freddie Mac.

The money managers are pushing for Congress to step in as they did with the corporate-bond market in 1939, when the Trust Indenture Act created new duties and responsibilities for debt administrators in order to protect investors and give them the confidence to extend credit following evidence of misdeeds after the stock market crash of 1929. Pimco, Schroders and BlackRock Inc. are among asset managers that want something similar for mortgage securities that suffered about $450 billion of losses through 2012.