Sen. Harrison A. William Jr. (D-N.J.) "sold his office" for the promise of riches from a phony Arab sheik, and then lied repeatedly on the witness stand when faced with undeniable incriminating statements on videotape, the government's chief Abscam prosecutor charged today.

In a biting, 2 1/2-hour final argument to the jury in Williams' bribery trial, prosecutor Thomas P. Puccio ridiculed the senator's claims that his tape pledges to help get government contracts for a titanium mine were meaningless "baloney."

Williams' testimony was simply not believable, Puccio said. "What do you do when you're caught red-handed on videotape?" he asked. "Was the baloney in the speech he gave the sheik? Or was the baloney coming out of Sen. Williams' mouth as he was on the witness stand in this case?"

Puccio noted that the senator had taken part in several meetings with undercover FBI agents posing as the sheik's representatives without ever rebuffing their corrupt overtures. The senator testified that many of his taped statements were meaningless and the stock in the titanium venture was worthless.

"I ask you," Puccio said, "did the words become meaningless and the stock become valueless before or after the witness saw the videotapes?"

Williams, the 61-year-old former chairman of the Senate Labor and Human Resources Committee, is charged with bribery and conspiracy for allegedly agreeing to trade his influence in getting government titanium contracts in return for a $100 million loan and a hidden interest; in the mine.

Puccio also attacked the credibility of co-defendant Alexander Feinberg, a long-time friend of the senator. The prosecutor referred to Feinberg as Williams' "bag man."

During the afternoon, Feinberg's attorney, Harry C. Batchelder Jr., told the jury his client might have been stupid and foolish, and exercised "bad judgment" in agreeing to the inducements of the undercover agents. But he said the FBI manufactured the crime and entrapped Feinberg into making damaging statements for the hidden cameras.

Williams' attorney, George J. Koelzer, will make his final argument to the jurors Wednesday. He is also expected to ask them to consider whether the senator was entrapped.

During his summation, Puccio played parts of a June 28, 1979, tape on which the senator told the undercover agent posing as a sheik, that there would be no problem getting government contracts for the mine. The prosecutor called the tape the "heart of the government's case" and said the senator's explanation of that tape was "the testimony of a desperate man."

"No matter how much he twisted. No matter how much he turned. This is the tape Sen. Williams can't change. This is the meeting he can't take back, no matter what he says on the witness stand," Puccio said.

After playing a section on which the senator said he'd take the titanium matter to the president, Puccio froze the frame and turned to the jury. "Is that what a United States senator is sworn to do? Is that the business of the government, to use one's office to make money? . . . I submit to you it's not and that's precisely what he's doing. No matter which way he squirms . . . he's using his influence, his prestige to fill his pockets."

He then played part of the tape where the senator says "no problem . . . it will come to pass" in answer to a suggestion that he use his position to get contracts. "Is there any doubt about it?" Puccio asked the jury, as he paced back and forth in front of them. "Is there any doubt that Sen. Williams sold his office, his influence on this tape? No matter how he twists and turns, the tape doesn't change."