The Senate Banking Committee is expected today to approve the nomination of Joseph A. Grundfest to the Securities and Exchange Commission.

Grundfest, 34, has been counsel and senior economist of the Council of Economic Advisers since 1984 and was previously an associate in the Washington law firm of Wilmer, Cutler & Pickering. He will fill the commission seat vacated by Charles L. Marinaccio for a term that would expire on June 5, 1990.

Last Friday the White House sent the Hill the nomination of Edward H. Fleischman to fill the SEC's other vacant seat. Fleischman, 53, is a New York securities lawyer and a partner in the firm of Gaston Snow & Ely Bartlett. He has been a member of the American Bar Association's committee on federal regulation of securities for two decades. He would replace James C. Treadway Jr. for a term expiring June 5, 1987.

If both men are confirmed by the Senate, the SEC would have its full complement of five members for the first time since last spring.

CHANGING RICO . . . Unlike the Justice Department, the SEC supports pending legislation that would modify the 15-year-old Racketeer Influenced and Corrupt Organizations Act (RICO), a law originally designed to fight organized crime that has been increasingly used in civil cases involving allegations of corporate fraud.

RICO allows plaintiffs to sue for treble damages and to recoup their attorney fees. "Thus, RICO's civil liability provisions have turned virtually every securities fraud claim into a potential RICO claim," SEC chairman John S.R. Shad told a congressional subcommittee recently. The law's "provisions have been used repeatedly against business men and women in ordinary commercial litigation not involving a pattern of repeated criminal conduct."

He cited Justice Department figures showing that two-thirds of the civil suits filed under RICO have been based on allegations of securities, mail or wire fraud, while only 7 percent have been brought against organized crime figures; the American Bar Association puts the figures at 77 percent and 9 percent, respectively.

The SEC supports changing the name of RICO to the "Criminal Enterprises and Corruption of Enterprises Act," substituting the word "criminal" for "racketeering" in the statute and imposing a uniform statute of limitations -- the major points of a bill introduced by Rep. John Conyers Jr. (D-Mich.).

Conyers also wants to make unsuccessful plaintiffs pay defendants' attorney fees if their suits were not "substantially justified." He noted that the changes were approved by the Vice President's Task Group on Regulation of Financial Services.

Meanwhile, Rep. Frederick C. Boucher (D-Va.) has proposed an amendment that would require that a firm or individual be found guilty of criminal charges involving racketeering before a civil RICO suit could be filed.

The Justice Department has expressed "serious reservations" about the Boucher bill. Deputy Assistant Attorney General John C. Keeney noted last month that the government has used RICO to file civil suits against a Kansas bank and a Florida defense contractor.

MOVING UP . . . Richard K. Wulff, a staff attorney with the SEC since 1972, has been named chief of the agency's Office of Small Business Policy under Associate Director Mary E.T. Beach.