To a certain degree, this push by the agency to require funds to be more liquid and disclose the riskiness of their investments mirrors the regulatory demands that commercial and investment banks face in terms of higher capital requirements and restrictions on trading activities.

And bodies like the International Monetary Fund, the Bank of England and the United States Treasury have been warning about the increasing risks taken by mutual funds as they pursue higher yields in a world of rock-bottom interest rates.

“As banks become more regulated, there is a risk that the frailties of banking are replicated elsewhere — such as mutual funds with short-term or runnable liabilities investing in opaque credit instruments,” said Paul Tucker, chairman of the Systemic Risk Council and the former deputy governor of the Bank of England, who wrote one of the few comment letters endorsing the proposal. “When this kind of thing goes wrong the social costs are very high, so we hope very much that the S.E.C. does not retreat on this.”

Mutual fund companies and their defenders have taken up arms in numerous ways. They are challenging S.E.C. officials in public forums like last week’s industry gathering in New York (sponsored by the Practising Law Institute) and a larger conference next week put on by the Investment Company Institute, the main lobbyist for the industry.

They have posted a stream of comment papers on the S.E.C.’s website, explaining why these proposals would do more harm than good. And they have pressed their case in person, meeting at least 15 times with agency staff since last fall, according to regulatory filings.

The essence of the industry’s complaint is that the idea of segregating securities in a portfolio on the basis of how many days it would take to sell them is a fool’s errand. Liquidity, or finding a buyer for what you want to sell, is one of Wall Street’s foggiest and most subjective notions. To guess how many days it would take to sell a leveraged loan or a corporate bond from Brazil is next to impossible, experts argue.