When the definitive history of the Reagan administration is written, assign David A. Stockman a major role. It appears more and more that Stockman, through his celebrated confessions to William Greider, has tagged the administration with a label that seems likely to endure.

Stockman's use of the expression "trickle down" to describe the president's so-called supply-side economic program has stuck. The phrase, in political/economic use decades before Stockman repopularized it, now crops up in conversations almost everywhere. And now it appears so deeply associated with the Reagan approach that people use it without apparent recognition of its source. History will have to remedy that lack of credit.

"Trickle down" is mentioned by all sorts of citizens in their casual comments. It forms the stuff of jokes, of commentary about events, of a way to describe what is happening. However it's employed, the phrase carries a barbed political message. It evokes an unflattering class connotation of rich vs. poor. That, too, more and more is associated with the Reagan presidency.

Examples, gathered here in St. Joe:

Morning, on a farm some 10 miles outside town: "One of Reagan's big mistakes was his tax cut," the farmer says. "He had to favor the rich. He's a little bit wrong in his thinking you can trickle down. It's not going to happen that way."

Afternoon, lunch in the graceful old Buchanan County Courthouse: "That trickle down's not getting down to me," a county employe says with some anger.

Evening, over drinks in a lawyer's home: "Did you hear that Mark Russell joke, the one he told in Buffalo?" his wife says. " 'Now we have the trickle-down theory. Give David Rockefeller a lot of money and it will trickle down to Laurence.' "

What follows here is offered as primary source material from the American Midwest for a chapter in that forthcoming historical volume, "Ronald Reagan and the Trickle-Down Years."

Gathered in the room were leaders of the St. Joseph homebuilding industry. Recent years had been good to them. They all had prospered. Now their business has been devastated.

Tom Woods, for instance, has had to lay off 60 percent of his workforce. "We've had builders lose their own homes on foreclosures," he says. "I don't know what the rates are, but I'm sure foreclosures are at an all-time high. I'm the leading supplier of lumber and building materials to the homebuilding industry here, 100 percent to the homebuilder. I don't do any retail business whatsoever. As you can see from my figures there I'm not sending near the money back to Washington that I did in the past."

He handed over a sheet of paper containing his business data for the three-year period from May, 1979, to the end of last month. His sales are off 73 percent. The amount of wages paid has dropped 55 percent. Sales tax revenues he's paid to the state of Missouri are off 85 percent. The amount of federal income taxes he has withheld has dropped 63 percent.

They all had similar stories to tell.

Jim Summers, Summers Realtors, a broker:

"The situation has reached the point where I think we have no in-town buying at all going on at this point. The statistics will show that we're off about 60 percent in the total dollar sale."

David Eldridge, who builds upper-bracket homes ranging up to $200,000: "In the last three years my production has gone from roughly 15 houses a year to drop down to about five and then to one. This year it's zero."

Jim Holmes, David and Wayne Kerns, Reed Kline, Bob Cobb added their descriptions of deep trouble. The hardest thing, one said, was how their attitudes were being affected. "We all used to feel that we were winners, that we were in an economy that was fairly constant. Now it's upside down. From an emotional-psychological standpoint, there's a lot on the line. Are we winners or we ultimately going to be losers?"

They all had been strong supporters of Ronald Reagan, and still admire him personally. But when they were asked to assess Reagan and Reaganomics now, the tape recorder picked up these words:

* "As a nation as a whole we took a conscious approach through Reagan's leadership, and I voted for him the last time. I thought it would be good to reduce inflation. And I think the country as a whole felt that was a goal, but now we've overkilled it. We're in a lot worse situation now than we were three years ago when we had inflation that was double digit, 13 percent or so in '79, and it's probably 5 or 6 percent now, but the whole economy can't operate."

* "I think Reagan's main policy was to bring down inflation at the same time he cut taxes and raised military spending. From an ideological standpoint, if it had worked it might have been fine. But it certainly is not trickling down to this level."

* "I voted for him. I thought a lot of his policies, his ideas. Given a chance, I thought they would improve things overall. But like so many other people he doesn't get the opportunity to see the plans executed. I think the whole game is political and maybe it's beyond what I can understand."

* "I also voted for the man and thought his programs would be a lot better than what we'd seen in the past. It was the approach on the supply side that I had great hopes for. Downrange, into the program, we have not seen that great an effect. In fact, it's been a worsening. The future? I don't know exactly what should be done. The deficit is a major problem and, if given a chance to really work all the way through, I think it would have come on through, but, we've overkilled."

* "I agree totally with that. Sometimes the medicine is extremely bitter to take, yet if the medicine kills you who's going to win in a situation like that?"

Something else Stockman said is having impact here. These are not old confessions, as told to Greider, but recent words of his from Washington. To beleaguered businessmen here, they are especially disturbing.

"There was an item in the paper the other day quoting David Stockman as saying that last year the federal government absorbed 34 percent of the available credit in the country and now for the fiscal year '82 they're going to absorb 56 percentage of the total available credit," one of the realtors remarked. "Knowing David Stockman and the federal government, I would say that's probably a conservative estimate."

A banker, who also cited that Stockman remark, commented, "If we're going to have 56 percent of available funds next year sapped up by government to finance this deficit, and those are the figures you're beginning to see, we've got some real problems out here. Obviously, something haywire has happened."

Indeed, it has. If that report of Stockman's is correct, our problem isn't over a lack of wealth trickling down. Our problem involves stopping a hemorrhage of the nation's economic blood supply.