Sen. Charles E. Grassley of Iowa, the top Republican on the Senate Judiciary Committee, is asking the Department of Education to explain its policies for dealing with investors seeking to profit from nonpublic information. In a letter sent today, Mr. Grassley suggests that short-sellers—investors who bet, in effect, that a company’s stock price will go down—may have influenced the department’s “gainful employment” rule-making. It is an argument that opponents of the rule have been making for weeks.