The Senate Ethics Committee yesterday named a Washington lawyer, Robert S. Bennett, as special counsel for its Abscam investigation of Sen. Harrison A. Williams Jr. (D-N.J.).

An effort by Williams to add former Senator Robert Morgan (D-N.C.) to his defense team was stymied, however, by a potential conflict.

The committee voted earlier this month to file charges against Williams, 61, the former chairman of the Senate Labor and Human Resources Committee, because of his May 1 conviction on bribery and conspiracy charges.

He and co-defendant were found guilty of agreeing to trade his influence in getting government contracts for titanium in return for a promised $100 million loan from an FBI agent posing as an Arab sheik.

Bennett, 41, recently served as a legal consultant to the Senate Foreign Relations Committee's nomination hearings on Secretary of State Alexander M. Haig Jr.

Sens. Malcolm Wallop (R-Wyo.) and Howell T. Heflin (D-Ala.), chairman and vice chairman of the Ethics Committee, respectively, said they couldn't predict when a disciplinary hearing would be held.

Williams held up announcing that Morgan would participate in his defense after receiving a letter from Wallop warning that Morgan's services might be restricted by Senate rules against a former member lobbying for a year after leaving Congress.

Morgan, who lost his reelection bid last fall, was a member of the ethics committee, and thus was privy to its early discussions of the Williams' Abscam case.

The House has expelled one member and has forced two others convicted in Abscam to resign. Williams has been lobbying his colleagues over the past few weeks, attempting to convince them he shouldn't be expelled before a full airing of his complaints about the conduct of government investigators in his case.