Senate leaders yesterday agreed to postpone until early next year debate on whether to expel Sen. Harrison A. Williams Jr. (D-N.J.) because of his Abscam bribery conviction, according to Senate sources.

The agreement was made at the request of Democrats after Sen. Daniel K. Inouye (D-Hawaii) volunteered to represent Williams on the Senate floor but said he would need a month to prepare his case, leadership sources said. Debate had been scheduled to begin Thursday.

The move means that a vote on Williams' expulsion would be unlikely until after Jan. 19 when newly elected Republican Thomas Kean becomes governor of New Jersey.

If Williams, a liberal Democrat, is expelled after that date, Kean would appoint his successor. This opens the possibility that Republicans could increase their Senate majority from 53 to 54 members.

Fearful of the political fallout, Republicans had been reluctant to seek a delay on their own. But Senate Majority Leader Howard H. Baker Jr. (R-Tenn.) agreed to put off debate after meeting with Senate Minority Leader Robert C. Byrd (D-W.Va.) and Inouye, Senate sources said.

Baker and Byrd had prepared to announce the move yesterday, but pulled back when they were unable to pursuade Sen. Howard T. Heflin (D-Ala.), the ranking minority member on the Ethics Committee, to immediately go along with the deal, according to sources. An announcement is expected today.

The agreement came as pressure was building from Democrats, who were uneasy about disciplining a colleague and uncertain about whether Williams had received due process under law.

It is the latest in a series of delays dating back to May 1 when Williams was convicted of nine counts of conspiracy, bribery and related charges for agreeing to use his influence to get government contracts for titanium for a $100 million loan from an undercover FBI agent posing as an Arab shiek.

No senator has been expelled for misconduct since 1862 when three were ousted for aiding the Confederacy.

Williams, 61, is a former chairman of the Senate Labor and Human Resources Committee. Until yesterday he had trouble finding another senator to represent him during debate.

Inouye said he offered to represent Williams because "he deserves his day in court."

"I'd like to see the Senate give this matter the full attention it deserves," he added.

Sen. David H. Pryor (D-Ark.), a member of the Ethics Committee, said Inouye "is doing a very courageous thing. He is a man the Senate respects."

Williams, a 23-year veteran of Congress, is one of seven lawmakers convicted in the Abscam investigations.

The Senate Ethics Committee voted unanimously to expell Williams Aug. 24. He is the first incumbent senator to be found guilty of a criminal charge since 1905.