What Mary Schapiro considered her most important task had just run aground, a symbol of the aspirations and missed opportunities of her tenure as head of the U.S. Securities and Exchange Commission.

Schapiro worked for two years on a plan to head off what she calls the "terrifying" prospect of a run on money-market mutual funds like one that forced a U.S. rescue in 2008. After fellow commissioners refused to follow her lead, she teared up as she worked on a statement accusing opponents of having their heads "in the sand," two people involved in the process said.

It's not surprising that Schapiro's frustrations boiled over that August evening. She has told friends that the late nights and almost constant policy battles have left her exhausted and eager to depart after the November election.