The special counsel conducting the House ethics committee's Abscam investigation has been instructed to stop working on potential disciplinary cases against Reps. Raymond F. Lederer and John P. Murtha, both Pennsylvania Democrats.

John Swanner, the committee staff director, said the abrupt halt to the work of special counsel E. Barrett Prettyman Jr. early this month was caused simply by the end of his contract at the close of the old Congress. "There's no way to do it [extend the contract] and abide by the rules," he said. "I can't see that a couple of weeks is going to make that much difference."

Rep. Louis Stokes (D-Ohio), new chairman of the committee, was quoted in Congressional Quarterly last week as saying he might delay any committee work until the Rules Committee has acted on proposed procedural changes for ethics. But Stokes said yesterday that he plans to adopt committee rules at an organizational meeting next week.

Still, some members of last session's ethics committee expressed dismay at Stokes' failure to overcome the contract technicality so Prettyman's work, at least, could continue.

"I would hate like hell to give the public the impression this is going to be a 'go slow' period for congressional ethics," said Rep. Richard Cheney (Wyo.), new chairman of the Republican Policy Committee.

Echoed Rep. Harold C. Hollenbeck (R-N.J.): "We shouldn't appear to be dragging our feet." And Rep. William Thomas (R-Calif.) said he was concerned that the committee might not deal with the Lederer case quickly, as it did last fall when then-Reps. Michael (Ozzie) Myers (D-Pa.) and John Jenrett (D-S.C.) were convicted of taking bribes in the FBI's Abscam undercover investigation.

Myers was expelled from the House, the first such action since the Civil War. Jenrette resigned rather than face disciplinary proceedings. Lederer was convicted on similar bribery charges this month. He was the only one of six House members charged in the Abscam cases to be reelected in November.

Murtha was not indicted, though he was secretly videotaped discussing a payoff with undercover agents.

Stokes said he plans to continue the investigation as soon as the committee is reorganized.

Prettyman said he believes "both the public and the Congress are entitled to have the committee make a determination up or down [on the remaining cases] just as quickly as possible, with all due regard for a member's due process rights. The difficulty is that the committee won't be able to move quickly because the work will not have been done." In a memo to the committee Jan. 12, Prettyman noted he was about two-thirds done with the Murtha case, but had not started the Lederer case review when he got the stop-work order Jan. 3.

The committee, known formally as the Committee on Standards of Official Conduct, long had a reputation of unwillingness to pursue allegations of colleagues' misconduct aggressively. But last year, the committee, under then-chairman Charles E. Bennett (D-Fla.), moved quickly to discipline the members convicted on Abscam charges.

Stokes, however, had a record of opposing almost every effort to act swiftly on disciplinary proceedings. He said his votes were based on sincere concerns about the due-process rights of accused colleagues. Thus, he added, he personally opposes committee Rule 14 that requires immediate action when House members are convicted of felonies.

The rule was adopted to prevent a recurrence of the nine-month time lag in proceedings against Rep. Charles Diggs Jr. (D-Mich.) after his 1978 conviction on a payroll kickback scheme.

Stokes said he felt insulted by members' implications that his tenure as committee chairman might be a "go slow" period for policing members' conduct. "I haven't even had my first meeting yet. How can they condemn me now? Because I'm a black chairman?"

Cheney said, "I have no reason to believe there's anything other than a straightforward explanation [for ordering Prettyman to stop work]. But it looks bad, and the committee has an obligation not to let down on this investigation."