It was late Saturday afternoon, and the United States Senate, or, more precisely, the handful of senators who couldn't come up with an excuse to be away - was still in session.The few senators on the floor were inattentive and irritated that the majority leader had called the Saturday session to accommodate the rush of last-minute legislation.

It was, Adlai E. Stevenson realized, the perfect setting for an end-of-session coup.

Stevenson, a Democrat from Illinois, was present because he was floor manager of the pending business, a routine bill extending the life of the Export-Import Bank. The coup Stevenson had in mind, however, had nothing to do with the bank.

At any other time, such a transparent ploy would have died instantly. But in the confusion of a Congress that is about to close, such coups slip through by the dozen.

Stevenson sent to the desk a lengthy amendment he had prepared, then launched into a brief speech explaining that it dealt with a dam in his native state. When Sen. Ted Stevens (R-Alaska) looked up with a quizzical eye, Stevenson went over to set him straight.

Stevens, the assistant minority leader, was present to protect Republican interests on a day when most of his GOP colleagues were off on the campaign trail. He merely wanted assurance that Stevenson's complicated amendment wouldn't upset any minority senators.

No problem, Stevenson said. The amendment was not at all controversial. It was "virtually identical" to a bill that had passed the Senate before, he said, and no Republicans would mind if it went through again. With that guarantee, the Republicans let Stevenson's amendment pass by voice vote.

That voice vote, which was far more consequential than Stevenson had let on, three another obstacle into the complicated congressional path of S.-790, the waterway user charge bill, which had finally seemed to be nearing the end of its long odyssey through Congress.

Stevenson's amendment guaranteed authorization of funds to rebuild Lock and Dam 26, a major facility for commercial barge lines on the Mississippi River at Alton, Ill. Since the start of the 95th Congress, that authorization has been one of the most controversial public works projects in the nation because it has been tied to S790, the bill that would impose, for the first time in history, fees for barges using inland waterways built or maintained with federal funds.

The linkage between the Alton dam and the waterway fee was the parliamentary brainchild of Sen. Pete V. Domenici (R-N.M.) the sponsor of S.790, and it has served his purposes beautifully.

Although the barge industry, which has never paid a penny for its use of the government's multibillion-dollar waterway network, is devoutly opposed to any waterway fee, it is also devoutly behind the effort to rebuild the Alton Dam. By linking the two measures, Domenici had forced his bill's strongest opponents to lobby, if reluctantly, for its passage.

Stevenson's coup on Saturday, while Domenici was in Albujuerque running for a new term, torpedoed the New Mexican's strategy. For Stevenson's amendment authorized the Alton Dam without imposing a waterway fee. In effect, it disconnected the motor that was driving S.790 toward final passage.

Domenici was furious when he heard about Stevenson's ploy, but he couldn't be entirely surprised. Trick plays and oblique angle shots are common - in the hurly-burly of a Congress that is about to close.

Members who have failed over the past two years to win passage for their pet bills now have only 10 days left to do so. As a result, they are on the alert for any vehicle that might pull the bills into law.

In the Senate, the Export-Import Bank authorization is an almost irresistible target. The bill has already passed the house, and seems sure of presidential approval. There is no time limit for debate on the measure, and thus no requirement that amendments have any logical connection with the basic bill.

As a result, the bank authorization has turned into the last "Christmas Tree" bill of the 95th Congress. It has been ornamented with proposed amendments ranging from textile trade and Indian affairs to a sweeping revision of the Internal Revenue Code.

When a group of senators wanted a resolution condemning wage and price controls, they struck it on the Export-Import Bank bilL. When Sen. Edmund S. Muskie (D-Maine) decided on one last try this year to pass his "sunset" bill to phase out federal agencies, the bank authorization was waiting there for him.

All of this extraneous material has come up when senators have other things on their minds. There are elections to win and vacations to start and junkets to plan, and there still remain to be considered some of the most important bills of the 95th Congress.

In that setting, Stevenson could perhaps be excused for fudging things somewhat when he brought up his amendment to the bank bill late Saturday. Although he assured the others present that a "virtually identical" amendment had passed the Senate previously, in fact, a virtually identical amendment proposed by Stevenson in June of 1977 was defeated by a 51 to 44 vote.

Stevenson's coup added more confusion to the confused parliamentary history of s.790. Domenici headed back to Washington determined to reverse it. His fellow Republicans resolved to see that Domenici did.

There were only 10 days left for the 95th Congress, and the fate of the waterway fee was still unclear.