On the eve of the debate on whether to expel Sen. Harrison A. Williams Jr., the Senate majority and minority whips yesterday called for a full-fledged Senate inquiry into "abuses of power" by the Executive Branch that they think have led to unjustified investigations of members of Congress.

Minority Whip Alan Cranston (D-Calif.) also indicated that he will propose a Senate censure of Williams (D-N.J.) rather than the resolution of expulsion voted last August by the Senate Ethics Committee.

Under the milder censure, Williams would have the option of serving out his term or resigning rather than being expelled. A two-thirds majority of those present and voting is needed for expulsion; a censure motion requires only a simple majority.

Cranston and Majority Whip Ted Stevens (R-Alaska) said at a joint news conference that they would introduce their "abuses of power" resolution Thursday. They said they would not seek action on it until after the Senate has voted on Williams, because they think it goes beyond his case.

If the Senate approves the inquiry, it is expected to be referred to a standing committee, such as the Judiciary Committee.

Senate Majority Leader Howard H. Baker Jr. (R-Tenn.) "indicated to me that he did not want another specially created Watergate-type committee," Stevens said.

Cranston indicated that he would prefer censuring rather than expelling Williams, and left no doubt that he hopes his proposed inquiry will influence the outcome of Williams' case. "The matters are inextricably woven together," he said.

Cranston said he is convinced that a Senate investigation of the way the FBI and perhaps the Internal Revenue Service have targeted lawmakers in the past would reveal "abuses of power of the magnitude of Watergate" in almost all respects, except that of presidential involvement.

Stevens, more cautious, said he had not reached the conclusions Cranston has, but said he thinks it is important to determine how the FBI decided which lawmakers to approach in the Abscam undercover bribery and influence-peddling investigation.

Six House members also were convicted in Abscam; one of the six was expelled, and the others either resigned or lost reelection.

Cranston and Stevens said they are worried, in the words of Stevens, that "the Executive Branch could develop a pattern of dealing with members of Congress in a manner so as to influence their conduct either intentionally or unintentionally."

Stevens said he was not trying to suggest that members of Congress should be immune from investigation by Executive Branch agencies, but he suggested that such investigations ought to be prefaced at least by a report that the member in question "has violated the law."

In at least some of the Abscam cases, Cranston said he was satisfied that lawmakers "were set up as targets" and tempted to commit crimes without the FBI's having any prior reason to suspect them of a "proclivity to commit crimes."

Asked why he was getting so exercised about Abscam now, on the eve of the Williams debate, Cranston replied: "You know how it works around here. You focus on what you have to focus on. The looming Williams case forced me to focus on what's been happening here."

The Senate, always proud and self-protective, has not expelled anyone since 1862 when Jesse D. Bright, an Indiana Democrat, was thrown out for conspiring to sell guns to the Confederacy.

Williams' expulsion has long been considered almost certain. "Right now if there was an up and down vote, I think he'd be expelled," Sen. Daniel K. Inouye (D-Hawaii), the World War II hero who volunteered to lead Williams' defense, said yesterday.

But at times a certain dynamics takes over the Senate, an immensely personal and clubby institution.

Sen. Howell Heflin (D-Ala.), vice chairman of the Senate Ethics Committee, said a move to censure "could very well be successful. The requirement of 67 votes needed for expulsion if all members vote is pretty protective to a senator."

The debate will take place under extraordinary circumstances. All committee meetings are banned during debate, and Senate Minority Leader Baker asked that all senators remain on the floor.

"I urge, I even insist, members attend, listen and remain," he said yesterday.

For the most part, senators have talked about Williams only in hushed and guarded tones. "This is something each senator has to decide on his own," said Sen. Bill Bradley, the junior Democratic senator from William's home state. "It is not something senators talk to one another about."

The delays have caused some bitterness, and probably benefited Williams. "I don't believe the delays have made the Senate look well," said Sen. Malcolm Wallop (R-Wyo.), chairman of the Senate Ethics Committee. "I believe that we have been waiting on things that are irrelevant to the case."

Williams, 62, is charged by the Ethics Committee with bringing "dishonor and disrepute" to the Senate for agreeing to trade influence for a hidden share of a $100,000 million loan from an undercover agent posing as an Arab sheik.

The affair was captured on secretly recorded videotape. Williams was convicted May 1 on nine counts of bribery and conspiracy, and sentenced to three years in jail. He has appealed his conviction.

The tapes, which have been viewed by 85 senators, show Williams discussing with the "sheik" how he could use his influence to get government titanium contracts, and how he could hide an 18 percent interest in a titanium mine to be developed along Virginia's Piney River with the sheik's loan.

Williams, once a Senate power as chairman of the Labor and Human Resources Committee, has consistently maintained his innocence, and refused advice from Senate leaders that he resign.

During the Ethics Committee hearings, he said that he "may have been guilty of errors in judgment" and "excessive boasting and posturing" but "I never engaged in any illegal conduct; I never corrupted my office."

After the 6-to-0 vote against him in the committee, Williams changed lawyers and press secretaries. He was tutored in how to appear more effective before television cameras, and contended that he was the victim of a FBI conspiracy because of his support of Sen. Edward M. Kennedy (D-Mass.) over Jimmy Carter for the presidency.

Debate on the expulsion resolution is expected to take no more than two or three days.