Sen. Harrison A. Williams Jr. (D-N.J.) told at least one colleague last night that he is ready to resign rather than face almost certain expulsion from the Senate today.

But one knowledgeable source cautioned that Williams is in "a highly emotional state" and his decision might not be final.

The source, a senator and a friend of Williams who asked not to be identified, said he wouldn't be surprised if Williams resigns today as the Senate prepares to vote on his censure or expulsion. Williams, 62, was convicted of bribery and conspiracy last summer in the government's Abscam investigation.

The word of the decision came as Williams, a 23-year-Senate veteran, told a news conference, "I'm still intent on making the case as I know it." Earlier he had said, "Right now, I'm not thinking resignation."

But as the Senate ended its fifth day of debate on a resolution to expel Williams, his chief defender, Sen. Daniel K. Inouye (D-Hawaii), said he was certain that Williams was considering resigning.

And Senate Minority Whip Alan Cranston (D-Calif.) said it would be impossible for Williams to avoid expulsion.

The Senate had been expected to vote late yesterday on a Cranston-sponsored resolution to censure rather than expel Williams, but the vote was delayed in part because a number of Democrats had urged Williams to resign, sources said.

Williams received a series of setbacks during debate yesterday as his fellow New Jersey senator, Bill Bradley, and four other Democrats announced that they would vote to expel him.

Bradley, in a dramatic moment near the end of the debate, said he was sad to say that he would vote against a colleague who had offered him so much "help and friendship" as a new senator.

But Bradley said he had seen and heard Williams on tape, and "for whatever may have been the inducement or entrapment, Sen. Williams' did not meet the minimum standards expected of a United States senator, and the Senate must act."

Bradley, who for months had refused to take a position on Williams' expulsion, was joined in rapid-fire order by Democratic Sens. Patrick J. Leahy (Vt.), John C. Stennis (Miss.), George J. Mitchell (Maine) and David H. Pryor (Ark.), who as a member of the Ethics Committee voted for expulsion last summer, but later expressed deep doubts about the case.

The senators who spoke out yesterday--significantly, they are not members of the ethics panel--showed obvious discomfort.

Williams "was washed up on our shores with both arms broken and both legs broken," Pryor said. "And now we are asked to cut off his head and rip out his heart. It is not an easy moment for the Senate."

Stennis, long a symbol of Senate propriety, said he was amazed and hurt by disclosures about a man he admired. "He was always traveling the wrong road," Stennis said of Williams. "He did not make tracks the other way and stay away. Instead, he went back again and again."

Stennis, Pryor and Leahy attacked the behavior of the FBI and the Justice Department in the case.

Leahy, a former prosecutor, described Abscam as a "slimy, sleazy operation to entrap someone," and pledged to join others in demanding an investigation of "this reprehensible, inexcusable" activity.

When Leahy had finished, Williams, his face red, rose to challenge his colleagues' decision on expulsion and, as he has done repeatedly, claimed innocence of any wrongdoing.

"To avoid looking at the government's actions is to look at only half the case," he said.

Williams was convicted of agreeing to use his office in exchange for a hidden share in a titanium mining venture to be financed by a $100 million loan from an undercover FBI agent posing as an Arab sheik. Williams was sentenced to three years in prison and a $50,000 fine.

In a powerful closing defense argument, Inouye called the conviction "a house of cards that could collapse at any moment" and leave the Senate "looking foolish" if the conviction is reversed on appeal.

Inouye said he had received telephone calls from other senators after Williams spoke in his own behalf last week. "They told me Pete blew it," Inouye said. "He shouldn't have talked so long. He rambled . . . . He should have been more humble. Jeanette Williams' wife shouldn't have gone on television."

His voice booming, Inouye continued, "Are we to punish him because his presentation was rambling, not in the tradition of Daniel Webster? Are we to punish him because his wife believes in him?

"Let's not punish Pete Williams because he's caused us pain and embarrassment, or because editorials urge us to do so. Or because it is a politically expedient thing to do. Or because he has not had the good grace to resign."

Sen. John Melcher (D-Mont.), a longtime friend of Williams, accused leaders of the Ethics Committee of repeatedly "misstating the law and the facts of the case" in their presentations on the Senate floor.

"I am not ready to vote for expulsion because I'm not sure of all the facts," he said, later adding, "I will not board a railroad train to a due-process holocaust."

Williams was the only senator among seven members of Congress convicted in Abscam. None of the other six remains in the House. John Jenrette (D-S.C.) and Raymond Lederer (D-Pa.) resigned before the House completed an expulsion move in their cases. Michael (Ozzie) Myers (D-Pa.) was expelled, and John Murphy (D-N.Y.), Frank Thompson (D-N.J.) and Richard Kelly (R-Fla.) were defeated in reelection bids.

Williams' government pension, about $45,000 a year, will be the same whether he resigns, is censured or is expelled, according William F. Hildenbrand, the secretary of the Senate. The amount is based on length of government service.