The elfin fellow with the enigmatic smile is down there again on the Senate floor, driving Republicans out of their tree, which must mean that farm legislation is pending again and that John Melcher of Montana is up to his old tricks.

As often happens to others in this curious city, Melcher has somehow got a reputation -- not necessarily pejorative -- as an obstructer of farm legislation and as a man of unfathomable motivation.

But in another sense, as the Senate debates a new farm bill, the Democrat from Montana is all that stands in the way of a Reagan administration steamroller that wants to squash federal farm spending and reshape the rural countryside.

Once again, as has happened in the past, the rules of the Senate have given Melcher a whip hand in the debate and the Republican leadership is pulling out every parliamentary stop it can find to sit him down.

Majority Leader Robert J. Dole (R-Kan.) has denounced him on the floor, blamed him for thwarting passage of the bill and called into question the sincerity of his professed aim of aiding farmers. Agriculture Committee Chairman Jesse Helms (R-N.C.) sputters and gets emotional when Melcher starts talking.

Through it all, Melcher cracks a small smile that never lets on what he is really thinking, and just keeps coming back at them. He did it again last week and, sure enough, the Republicans were dispeptic.

The background is that Melcher led a campaign in the committee to stop the administration's plan to cut income supports, known as deficiency payments, that are paid to grain, cotton and rice farmers. With help from GOP defectors, Melcher got the panel to adopt a four-year freeze on the subsidies instead of the one-year freeze the White House sought.

Back in the summer, when it appeared Melcher had the votes to carry the day, Helms refused to reconvene the committee to allow the inevitable vote. Then the Senate left on August recess and did not confront the Melcher four-year freeze again until mid-September.

But the freeze carried and went to the floor as part of the farm bill, despite efforts by Dole and Helms to get the committee to reverse itself. Things have been nasty ever since.

On the floor last week, Dole and Helms flexed their muscles a couple of times, trying to upset Melcher, but it didn't exactly work. They railed at Melcher for backing budget-busting legislation, but then blocked his bid to call up an amendment that would have trimmed $7.6 billion from the farm bill.

They lost, 51-48, when they tried to pass their one-year freeze on subsidy payments. Through a complex parliamentary move, Dole regained control and got the Senate to support him on a test vote that left a substitute, with the one-year freeze, as the pending business.

Through it all, John Melcher just smiled.

Dole denounced his budget cut ideas as "smoke and mirrors." Melcher smiled.

When Dole put some of the Melcher ideas in his own substitute, Melcher forced him to admit they were the same smoke and mirrors. Melcher smiled again.

"There are other places in the farm bill to save money, not just by cutting target prices," Melcher said in an interview. "They have dealt with target prices as a crucial part of the bill and I thank them for it. It is a symbol to the rural community whether we are going to do something for farmers. They have handed us the high ground and I thank them for it."

The issue is important enough to the GOP leadership that Dole has threatened senators from the cotton and rice areas -- where deficiency payments tend to be highest -- that the subsidies will be cut if they do not come out for the one-year freeze.

Melcher is smiling, in part because he has been there before -- and stopped the seeming juggernauts. At least four times since 1981, with farm-related legislation hanging fire on the floor, Melcher has thrown up roadblocks because he did not like what was happening.

The odd thing about this, considering Melcher's reputation as a blind defender of the federal farm suppport programs, is that he actually does not like them.

"I have a problem with the whole scheme," he said. "Why do we have to have a program that pays farmers in lieu of them getting a price from the marketplace? The only reason I vote for these things is to keep them going until we come to our senses in this country. Why don't we find other mechanisms?"

Another tag they keep hanging on Melcher is that he is obsessed with ideas for sending more surplus food to the needy around the world. Mention "food aid," and a Melcher dissertation is sure to follow.

He probably is the foremost congressional champion of feeding the hungry in other nations. The specter of American grain and dairy products going unused drives him batty.

"If I were dictator for a day? I would set up exports, provide more impetus on exports, and my policy would be to try to turn our production machine loose to try to meet the world's needs," he said.

"This administration is blatant in its belief that there are too many farmers in this country. There really are not. There simply isn't too much food in the world. We're blessed in the variety of foods we have and there's just a breakdown in supplying that food to people who need it.

"And I say that's a hell of a lot more important right now than whether we have Reagan's 'Star Wars' defense or MX missiles."

Back to the farm bill, Melcher is smiling that smile again. "I don't care what Dole does to me. I don't mind his verbal assaults. But, obviously, he's been doing the stalling around here. . . ."

Will Melcher block it in the end? Will he talk the bill to death? He's smiling again, and the enigmatic plainsman is riding again. "The rules allow you to express your judgment," he said. "And sway the vote one way or another."