In the last legal action before he faces a Senate expulsion vote next week, Sen. Harrison A. Williams Jr. (D-N.J.) was sentenced yesterday to three years in prison and fined $50,000 for his bribery-conspiracy conviction in the FBI's Abscam undercover investigation.

U.S. District Court Judge George C. Pratt delayed the sentence until Williams' appeals are completed. He is the first incumbent senator since 1905 to be convicted on a criminal charge.

A Senate Ethics Committee aide said yesterday that Majority Leader Howard H. Baker Jr. (R-Tenn.) hadn't set the ground rules for the expulsion debate except to decide that no committee business would be scheduled. The last senator expelled was for treason during the Civil War.

Six House members also were convicted in the Abscam cases. Pratt sentenced three who were tried in his court last August to three-year prison terms and $20,000 fines. One, Michael (Ozzie) Myers (D-Pa.), was expelled from the House. None is yet in prison and none is expected to serve more than 20 months.

Williams was convicted last May 1 of agreeing to trade his influence in Congress for a hidden share of a $100 million loan from an undercover FBI agent posing as the representative of a fictitious Arab sheik. He attended seven meetings over a period of months with the undercover agents.

His encounters with the agents were recorded on videotape, as were those of the House members caught in the sweeping congressional corruption scandal. All the House members except Myers were defeated or resigned from office. Only Williams, 62, has continued to fight the judgment of his peers.

Williams repeated his claims of innocence again yesterday before the sentencing in the federal courthouse in Uniondale, N.Y. "I leave this court knowing, feeling and knowing, that I'm innocent of the crimes charged," he said.

He called Abscam a "sordid, manufactured attempt to get me to commit crime." His appeals claim that the FBI illegally entrapped him and violated his constitutional rights. Pratt rejected such claims after post-trial hearings.

The 3rd U.S. Circuit Court of Appeals rejected similar claims by two Philadelphia city councilmen last week.

Thomas P. Puccio, the government's chief Abscam prosecutor, sharply criticized Williams' conduct in his pre-sentencing remarks to the trial judge yesterday.

In an apparent reference to the oft-delayed Senate expulsion vote, he said, "Ironically, he stands before you today still purporting to represent the people of the state of New Jersey, who by his greed he has deceived and betrayed."

Government attorneys had considered the Williams case the weakest of the Abscam group because, unlike the House members, he turned down a cash bribe offer and was prepped on what to say before a key meeting by undercover informer Melvin Weinberg, a convicted con man.

But the jury in the trial clearly didn't find Williams' testimony credible. Puccio charged yesterday that Williams "put his office up for sale" and then took the witness stand "to boldly lie," but "failed miserably."

Thus the senator had to "engage in a massive media campaign to try to avert attention from his conduct and put the prosecution and the system itself on trial," Puccio said.

Puccio has been the target of several critical columns by syndicated columnist Jack Anderson. Puccio has responded in court papers accusing the senator and the columnist of acting in consort.

George Koelzer, Williams' attorney, told Pratt that Puccio's complaint about a "massive media campaign" by his client was "offensive" because federal officials have admitted that the initial publicity about Abscam came from government leaks.

Puccio likened the corruption uncovered in Abscam to that alleged in the investigation of South Korean influence-buying in Congress in the mid-1970s. But there were no videotapes during the Korean investigation, Puccio said.

If history had repeated itself, he added, Williams might have been "at worst a footnote in a committee report, not standing here before you awaiting sentencing on serious felony charges."