The time had come, the secretary of transporation told the president, to get tough on the waterway toll bill. The administration's credibility was on the chopping block; they'd have to act, now, to avert the ax.

The weekly Cabinet meeting had just ended. As the business suits and briefcases filed out the Cabinet Room, Brock Adams buttonholed Jimmy Carter. You don't get that many chances at a president, even if you're secretary of transportation, and Adams knew his message for Carter couldn't wait.

The message: the waterway bill, a controversial measure that would impose, for the first time, a user fee on barge lines moving freight on inland waterways, was coming up for a final vote in the Senate - probably the fir week in February.

With Carter's backing, the Senate had passed S,790, which established a strong waterway fee that would recover a large share of the government's annual expenditure on waterways. But the House had passed a much weaker bill. Now the issue was back in the Senate, and Adams had threatened, in writing, that Carter would veto the bill unless the Senate voted a fee substantially higher than the House version.

The problem was that a lot of senators - Democrats, influential ones - didn't believe that threat. At the urging of the barge industry, they were insisting they would vote for the House bill and then pressure the president to sign it.

If that happened, Adams said, the administration's final deterrent in its dealings with Congress - a veto threat - would be fatally defused.

The president told the secretary not to worry. The White House, he said, would take care of things.

It was a little vague, Adams told his allies in the Senate, but it was reassuring. Almost from the day last October when Adams had written his letter posing the veto threat, a lot of senators had insisted they didn't believe him. Good Democrats like Russell B. Long (Lt.), Adlai Stevenson (III), and Warren G. Magnuson (Wash.) had been talking opening about "calling the president's bluff."

It got so bad that Sen. Gaylord Nelson (D. Wis.), an old friend, called Adams with a warning. "You've got a problem here, Brock," Nelson said. "If we pass the House bill, and Carter doesn't veto, your credibility is going to be mincemeat around here."

Adams' staff, too, had some moments, around the first of the year, when they feared a softening of backbone on the question in the White House.

They knew that Long had discussed the question with Carter himself, and then reported back to his colleagues that the president was not exactly breathing fire on the question. "He might veto, and he might not," Long told the other senators with a mischievous grin.

Without the White House, there was a degree of schizophrenia on the waterway toll. Frank Moore, the president''s to reconsider - he would need Long's help on a lot of things more crucial than a barge charge bill. But Stuart Eizenstat, Carter's domestic issue expert, was taking the hardest possible line on behalf of a tough waterway fee.

Last weekend,, Adams' own lobbying team had met in the White House with the varius factions, but nothing had been firmly decided.

That was why Adams felt the need, after last Monday's Cabinet meeting, to take up the matter with Carter himself. The presiiident's guarantee that something would be done made him feel a lot better.

Within hours after the meeting, something was done. Adams received a brief note on White House letterhead, signed by Richard G. Hutcheson III, Carter's chief paper shuffle. "Let our position be clearly known," Hutcheson quoted Carter as saying, "including possibility of veto."

It wasn't all Adams might have hoped for, but it was probably enough. The memo was quickly copied and dispatched to the Senate. It ought to scare those Democrats enough, the secretary thought, to make them vote for something more than the House-passed waterway fee.

In the Senate, meanwhile, Russell Long came down with the flu. The waterway bill, which had been scheduled for a vote on Thursday, would have to be put off until next week. Borck Adams would have to hang on a few more days.