Sen. Harrison A. Williams Jr. (D-N.J.) watched a summer rerun of Abscam yesterday. But this time six of his colleagues, rather than the Brooklyn jury that convicted him of bribery in May, were wearing the earphones and watching the FBI videotapes. And they were considering charges that could lead to his expulsion from their exclusive club.

It was evident that neither Williams, who was found guilty of promising to trade his influence for a hidden share in a $100 million loan from a phony Arab sheik, nor the six members of the Senate Ethics Committee enjoyed the show Sens. Malcolm Wallp (R-Wyo.), the committee chairman, and Howell Heflin (D-Ala.), the vice chairman, both noted in their opening statements that the investigation was painful.

"It cannot be disputed that the events which bring us here today are sad," Wallop said. "In my life to date, or at this point in time, I have never engaged in so difficult, so unpleasant a task as we undertake today," Heflin said.

In his statement Heflin almost eulogized Williams, describing him as a man of compassion who "has championed the cause of the elderly, the handicapped, the unemployed, the disadvantaged, the migrant worker." Heflin later told reporters that judging a colleague was especially distasteful because "there's a closeness, a feeling of togetherness here."

The committee is considering whether Williams' actions in Abscam violated rules on disclosure, conflict of interest, and conduct unbecoming a senator. Six House members also have been convicted of Abscam charges. One was expelled and two resigned after disciplinary hearings last year. Three others were defeated for reelection.

The Senate delayed its hearing until Williams' trial was over. Kenneth R. Feinberg, Williams' lawyer, argued yesterday that the committee should wait until the trial judge rules on pending motions charging government misconduct in the case before recommending sanctions. Wallop and Heflin, however, have said they see no reason for further delay, because the Senate is judging the 61-year-old Williams independently, according to its own standards.

The evidence was familiar from the month-long trial in Brooklyn. But the small hearing room on the sixth floor of the Dirksen Senate Office Building was filled most of the afternoon as committee special counsel Robert Bennett introduced the videotapes. The "single most important," he said, was of a June 28, 1979, meeting at the Key Bridge Marriott Hotel in Arlington.

There Williams met an undercover FBI agent posing as a wealthy Arab sheik, boasted of his influence, and said there would be "no problem" getting government contracts for a titanium mine that was under discussion. The senator later accepted an 18 percent stock interest in the mine.

Bennett is expected to finish his presentation today. He told the committee members they should ask themselves three questions as they watched their colleague on videotape: "I this the way a United States senator should act? Is this the way a United States senator should use his influence and power? Is this the way that the public trust should be nurtured and preserved?"

Feinberg won't present Williams' defense until July 28. But he made it clear yesterday that he will attack the government's conduct of the investigation, just as the defense did unsuccessfully during the criminal trial. He said the committee would be seeing only "the concluding act of a play, written, produced, filmed and paid for by federal authorities.