IN THE LAST SCENE of the last act in the Senate drama, Daniel Inouye of Hawaii, who had so valiantly volunteered to defend Sen. Harrison Williams of New Jersey from expulsion, achieved a touch of Shakespearean felicity.

He turned to the gray-faced senator who sat three rows to his right, and said: "My dear friend, I bid you farewell and Godspeed; to you and Jeanette, God bless you."

For a few moments, it seemed that the Senate would redeem some honor and dignity from what was called an "ordeal" but which had seemed to be more of an endurance test.

Democratic Whip Alan Cranston had, in a somber speech, imbued Williams' stubborness with a gloss of high purpose. If Williams, who had been convicted and sentenced in federal court, had not persisted well beyond the merits of his case, the gross misconduct of the FBI in the Abscam affair "might never have been brought to the attention of the Senate."

Cranston, who came to the cause in the final days, pointedly took his text from remarks of Sen. Thomas Eagleton (D-Mo.), who had rendered a harsh judgment on his old friend.

"Sen. Williams," he complained, "has not had the good grace and good judgment to withdraw from this body."

It was a verdict privately subscribed to by almost every other senator who had been trapped in the chamber for six days, whipsawed in an argument over who had behaved worse, Williams or the G-men. But privately, they were not sure that Eagleton, who in 1972 had taken seven days to withdraw from the Democratic ticket, after disqualifying disclosures, was the best expert on timely exits.

Cranston did not, of course, mention that. He simply said that Williams' fate had meaning, because he had by his "lonely fight" alerted the country to an "out-of-control, undercover operation" that cried out for investigation.

Williams had been extraordinarily well served by Inouye and Cranston. They had managed, fleetingly, to shift the discussion from the man on the incriminating videotapes to the men behind the hidden cameras.

He was also prosecuted by several of the most decent men in the Senate. Chairman Malcolm Wallop of Wyoming had brought new credit to the Ethics Committee. Sen. Howell Heflin (D-Ala.) had made the case against Williams in a majestic indictment that liberated the members from their dilemma of fearing that a vote against Williams would mean a vote for Abscam.

But Williams has an unerring sense of anti- climax and excess and soon plunged the Senate back into the embarrassment which had been its home this past week. He said grandiosely that he had done what he did because he "did not wish the Senate to bring dishonor on itself by expelling me." He got so wound up in quotations from the Bible, references to his clergyman-ally Carl McIntyre and his wife Jeanette, who were sitting together in the front row of the gallery, in quoting the Bible and protesting his innocence that he momentarily forgot why he had stood up in the first place.

"Protections must be built for the nation," he began in yet another flight. An aide tugged gently on his coat and brought him back to reality.

"I, er, have come to a decision," he said, as if voicing an afterthought. "I intend to resign."

It was over, except for the senators' flowery tributes to his courage and their own.

But the fact is that it did not take a great deal of courage to stand up to refusing to serve with a convicted felon. Nor will it take much courage to demand, as have Cranston and Republican Whip Ted Stevens of Alaska, a full investigation of the odious tactics of the FBI and its odious hireling, Mel Weinberg. Simple self-preservation requires an inquiry into a government crime school.

The senators, in the long run, will not be judged either by their investigation of the FBI. Another kind of courage is most urgently required from them. Will they have the nerve to stand up to another "out-of-control undercover operation"? That is the covert activity which Reagan is supposedly undertaking against Nicaragua. The secretary of defense and the secretary of state both refuse to confirm or deny a story in The Washington Post about a covert "destabilizing" operation, costing millions of dollars.

In 1975, the Senate voted specifically to forbid such dirty tricks in Angola. The country is now under a "Marxist" government which nonetheless has the blessing of David Rockefeller.

Will the Senate, having recognized the danger to itself in domestic cloak-and-daggery, insist that it has no place in our dealings with other nations, even those which irritate Alexander Haig?

Nobody died in Abscam. But if we repeat the Chilean venture in Nicaragua, we are in for blood in Central America and shame in the world. The Abscam case was a piece of cake compared to Nicaragua, and we will shortly know if the Senate, which knew what to do with Williams, has the stuff for something big.