Attempts to make sweeping changes to a popular type of mutual fund that played a central role in the 2008 financial crisis have been derailed.

The chairwoman of the Securities and Exchange Commission, Mary L. Schapiro, wanted to bring her vision for regulating money market mutual funds to a vote as early as next week. But Ms. Schapiro acknowledged on Wednesday evening that three of the five commissioners opposed her plan and said she was calling off the vote.

While opposition was certain from the two Republican members of the commission, the crucial swing vote, a Democratic commissioner, Luis A. Aguilar, said in an interview on Wednesday afternoon that he would feel comfortable voting only after significant further study of the industry.

Ms. Schapiro wanted the $2.6 trillion money market fund sector to start holding cash reserves or to let their share prices fluctuate, instead of promising to pay investors $1 for every $1 they put in, among other changes.