"Frankly, there were times when I thought we might lose that roll call," the senior senator from Louisiana said, trying hard to keep a straight face. "You know, the president was working hard on this waterway bill. He was calling senators from that airplance, that Air Force One. I mean, he was pressuring these boys."

Try as he might, Sen. Russell Long (D) could not hold back a contented smile yesterday as he recalled his victory Wednesday night when the waterway toll bill had come up for a final vote on the Senate floor. But the memory of that vote was sweet, and Long's smile grew broader the longer he talked about it.

In a pitched parliamentary battle with the Carter administration, Long had convinced the Senate to approve his own relatively mild version of the waterway fee, a new federal tax to be imposed on barge lines hauling freight on federally maintained inland waterways.

In backing Long, the senators had rejected a stiffer fee proposal offered by Sens. Pete Domenici (R-N.M.). They had done so even though President Carter had worked hard on Domenici's behalf - and even though Carter had threatened to veto the entire water projects bill, a measure filled with authorizations for senators' pet construction projects.

The President's intervention had worried Long somewhat, and for a few hours Wednesday morning he had thought he might lose. But in the end, the personal persuasion of Long and some of his senior Democratic colleagues had proven more potent than the president's veto threat.

"I talked to some senators on this, sure," Long said yesterday. "And we had some [committee] chairman on our side, and they talked to their [committee] members. But still, it's hard when you're up against the president."

Long overcame the presidential push by telling his colleagues that Carter was asking for too much.

"I think the senators know that we've come a long way on this waterway fee," Long explained. "You know, last year, I didn't want to give up one penny. Now we're agreeing to almost everything Sen. Domenici asked for. I just told the senators that the White House should be happy with what we've given them.

"Anyway, this whole issue, you know, it's really a fight between the barge lines and the railroads. The railroads want this barge tax so it will hurt their competition. But I kept telling people that wasn't fair.

By defeating the president on the Senate floor, however, Long set himself up for what could be a tougher battle; convincing Carter to sign a bill he has explicitly promised to veto. In the flush of victory yesterday, the easy-going senator seemed to think that would be easy.

"Now, why won't he sign it? He said he wanted a capital recovery tax, and we didn't vote for his capital recovery tax. That's right. But we passed a tax - sure, it was less than he wanted - and maybe we could just give our tax a new name, we'll call it a capital recovery tax, and then he can sign this bill."

Long, of course, would be a winner in any case. If Carter does sign the bill, Long will get the credit for limiting the waterway fee. If Carter vetoes it, there will be no waterway fee at all - which would suit Long even better.

At the thought of that, the Louisiana smile grew broader than ever.