After months of delay and soul-searching, the Senate yesterday began an emotional debate on whether to expel one of its members, Harrison A. Williams Jr. (D-N.J.), with a pointed effort to counter charges that he was lured into wrongdoing by government investigators.

"Put simply, Harrison Williams traded on his office," said Sen. Howell Heflin (D-Ala.), vice chairman of the Senate Ethics Committee, which unanimously recommended last summer that Williams be expelled.

"He traded on his name and position. He acted in an official capacity out of motives for personal financial gain. And, in doing so, he sullied both his reputation and that of this institution," Heflin said. The debate, cloaked in somber and historical tones, began against a backdrop of moves to censure Williams rather than expel him.

A vote to censure would "set the wrong standard within, and send the wrong signal without," Heflin said. "To merely censure our colleague would be to put the Senate stamp of approval on conduct which we all know, deep inside, to be wrong."

Expulsion requires a two-thirds' vote of senators present and voting. Censure requires only a majority.

Williams, 62, was sentenced to three years in prison for bribery and conspiracy, the third senator in history convicted of a felony while in office.

Yesterday's debate marked the first time in 40 years that the Senate has met to consider expelling a member.

Williams' colleagues did so painfully and reluctantly, with all the pomp the Senate reserves for such occasions. All committee meetings were canceled and senators were asked to remain on the floor throughout the debate, which could last until Tuesday.

"Rendering judgment on the conduct of a colleague and a peer is unpleasant and distasteful," Minority Leader Robert C. Byrd (D-W.Va.) said. But he added that the Constitution requires the Senate to "be masters of our own house."

"We, for the people, are the keeper of the flame," he said.

Williams' wife, Jeanette, sat in the gallery as Byrd spoke. Beside her were Carl McIntire, a radio evangelist, and Indy Badhwar, a reporter for syndicated columnist Jack Anderson. All three have defended Williams.

Williams, wearing a bright blue shirt, red tie and charcoal-gray suit, listened stoically. Before the debate he told a news conference on the Capitol lawn, "This case deals with a manufactured attempt by operatives of the government to try to ensnare me in a crime, and they failed. That will be my case."

Williams, a 23-year Senate veteran, and five congressmen were convicted on conspiracy and bribery charges in connection with the Abscam investigation. A sixth congressman was convicted of conspiracy and accepting a gratuity.

Williams was convicted of agreeing to trade influence for a hidden share in a mining venture that was to have been financed by a $100 million loan from a fictitious Arab sheik, who was really an undercover FBI agent.

Heflin, a former judge with a booming baritone voice, led in presenting the case against Williams.

"I have never engaged in so difficult, so unpleasant a task," he said.

Standing at a lectern in one corner of the Senate chamber, Heflin spoke for almost three hours, his voice never wavering. He attempted to answer, point by point, the arguments that are likely to be raised in Williams' defense, and to try to resolve any lingering doubts some senators may have about the case.

He was part philosopher, part prosecutor, part country lawyer. He defined public trust, citing Plato, in lofty terms. He talked about the care and fairness the ethics panel had shown in the case.

He outlined the evidence against Williams, saying most of it was "based on the words . . . uttered by Sen. Williams and recorded on video and audio tape."

Noting that many senators feared that "if they can do it to Pete, they can do it to me," Heflin asked, "Do what? Offer you a lucrative share in a multimillion-dollar financial enterprise in which you don't have to invest one red cent, in which you only have to use your influence as a senator to get government contracts, or federal grants or favorable tax benefits?"

Any senator who "really knows right from wrong," Heflin said, "would not hesitate to get up and walk--walk away from sleazy characters swearing like sailors, walk away from talk about sheiks and deals and hiding interest and protection and concealment . . . walk away, in other words, from obvious impropriety.

"But he didn't," Heflin continued. "He stayed. He discussed. He agreed. He promised. He pledged: to abuse his office, his public trust, for which now he must be expelled."

Speaking in Williams' defense, Sen. Daniel K. Inouye (D-Hawaii) noted that the Senate had expelled only 15 members in its history, all for disloyalty or treason. "Surely the conduct you are asked to judge here is nothing nearly so severe," he said.

He said the FBI and the Justice Department created a trap with Abscam, "then goaded and cajoled" legislators into it.

" . . . It could happen to any of us," Inouye said. He noted that at one point Williams had rejected a monetary bribe, saying, "No. No. No. No." Asked Inouye: "Four times. How many times must one say no?"

After yesterday's debate Minority Whip Alan Cranston (D-Calif.) said he would propose a censure resolution against Williams today.