It's a cruel choice -- almost as bad as making budget cuts that will satisfy a president and infuriate voters. But at least choosing between Sen. Harrison A. Williams Jr. (D-N.J.), and the Justice Department in the Abscam case is something within its control, so the Senate put it off.

The liberal New Jersey Democrat, the only senator among seven members of Congress caught in a videotaped FBI sting operation, was convicted of bribery last May. The Senate Ethics Committee called his conduct "ethically repugnant."

But the way it all happened--a year-long effort by undercover FBI agents impersonating sheiks, and in one case, a senator's niece--is equally "ethically repugnant" to senators, who don't know for sure that the FBI won't try something like it again, with a convicted felon deciding about their "predisposition" to criminal activity.

The general feeling was summarized by Sen. David H. Pryor (D-Ark.), a member of the Ethics Committee.

"You can't condone what Williams did," Pryor said, "but you can't condone what the FBI did either."

What especially bothers Pryor is "that horrid memo," an internal FBI document dated Nov. 27, 1979, in which an undercover agent expresses fear that the agents don't have enough to "get" Williams for his acceptance of shares in a titanium mine and must go after him harder. As a matter of fact, they subsequently offered him a bribe in an immigration case. The videotape shows him saying, "No, no, no, no."

The natural reluctance of the club to pass judgment on a fellow member is, in this instance, exacerbated by a wait for the other shoe. U.S. District Judge George C. Pratt has before him an appeal from Williams for a new trial because of what Williams calls "violation of due process."

Sen. Daniel K. Inouye (D-Hawaii), who volunteered to represent Williams on the Senate floor after no one else came forward, argued vehemently with Majority Leader Howard H. Baker Jr. (R-Tenn.) and Minority Leader Robert C. Byrd (D-W.Va.) that the Senate would look pretty foolish going for its equivalent of capital punishment if a new trial were granted.

Inouye, a war hero who lost an arm in combat, is much respected. He calls himself "a lousy lawyer" and offered his services to Williams because he thought Williams deserved his day in court. They vote alike 90 percent of the time but are not particular friends.

Inouye, who shone on the Watergate committee as a laconic, incisive questioner and spent Thanksgiving weekend reading the court record and the history of Senate expulsions, of which there have been 15--all he reminded the leaders after the appellate process had been completed.

"We would look like a bunch of self-righteous people trying to preserve our image in a rush to judgment," Inouye said.

Williams understands that his colleagues wish he would go quietly.

"This has been conveyed to me," he said on the telephone while waiting to hear if the Senate would delay his "trial" until January. "I sense things. I hear things."

What he hears, of course, is the muffled moans of Democrats, who see a Senate seat being lost. Republican Thomas Kean will take office Jan. 19 as New Jersey governor and, in the event of Williams' expulsion, would select a Republican successor.

In New Jersey, Williams is wished well and clapped on the back by voters who remember his exertions for unions and the poor.

"But," said a fellow politician who did not wish to be named, "they think he's a dead man. They raised all the money they could for his defense fund, but now they think there's nothing more they can do for him, and they want to save the seat for the Democrats."

Williams has worn an invisible shroud since the Abscam jury found him guilty. He moves about like everyone else, is greeted in the usual fraternal way, attends hearings, votes on the floor. With fellow senators he discusses everything but what is on his mind and theirs--his awkward situation. Now that Inouye has become his defense lawyer, the embarrassing embargo will be lifted.

Williams, a quiet man with a sonorous voice, has few enemies in the Senate. "I never heard him take a cheap shot at anybody," a colleague said.

Said Inouye: "If this was a guy with a reputation for being on the take, someone who was mentioned in criminal circles, it would be one thing. But the FBI comes along and throws enticement in his face."

As to why Williams was fingered by the FBI, some speculate that a change in his lifestyle on the occasion of his second marriage, to his striking blonde secretary, may have engendered the idea that he would be susceptible to government cash offers. The first Mrs. Williams, mother of the senator's four children, spent her time in New Jersey, and the senator rarely made the capital's party scene.

But senators find it abhorrent that Melvin Weinberg, a convicted con man, was allowed to shop around for candidates for corruption. They think the FBI should be setting up drug lords, not members of Congress. If Inouye has his way, the issue will be government misconduct, not the weakness of one senator.