JUST AS the House of Representatives rightly faced up to the unpleasant task of clearing its rolls in one way or another of members convicted in the Abscam trials, the Senate so far is moving with appropriate deliberate speed in judging the conduct of its sole convicted member, Harrison A. Williams (D-N.J.). The Senate Ethics Committee's action yesterday amounts to an internal version of an indictment -- based not on the legal questions that have been and still are before the courts, but on the question of whether the conduct of Mr. Williams has been consistent with a senator's sworn duty and responsibilities as set forth in the Standing Rules of the Senate.

The decisions of this committee -- and ultimately of the full Senate -- do not necessarily turn on whether a member's conviction is final or what any other congressional investigation may determine about the FBI's conduct in Abscam. The committee members' responsibility is to decide whether what their colleague has done calls for punishment and, if so, what the punishment should be. The range is from reprimand to expulsion.

As it stands, a jury has rejected the senator's version of events and concluded that he did try to exploit his office for personal gain. If the committee members reach a similar conclusion about the conduct of Mr. Williams -- as a member -- the behavior would constitute a violation of Senate rules and be ample cause for explusion.

Given the fact that the committee's staff has been reviewing evidence in the case for months, appropriately cautious consideration should not take all that long. It is awkward enough already for the Senate to be keeping in its number a member convicted of serious crime; and if that finding is coupled with one that the senator abused his office in the process, any punishment short of expulsion would require extraordinary justification.