The magic moment of the first National Rainbow Coalition convention came when the Rev. Jesse L. Jackson took stock of his audience at a breakfast speech. It was equally divided between white farmers and black city dwellers.

"Y'all a strange combination of folks sittin' in this room," Jackson marvelled, his voice thickened for the occasion with a down-home South Carolina accent. "A straaange combination!"

It was indeed strange. Not since the civil rights struggles of the 1960s have blacks and whites come together in significant numbers to forge an independent national political movement. And back then the whites, disproportionately, were members of the economic elite.

Jackson had something more daring in mind at the convention that wound up here yesterday. He wants to patch together a multiracial coalition of economic "outs" -- a populist constituency of anger whose very coming together, in its very improbability, would achieve one of its goals: to send them a message.

Consider: Several hundred farmers came halfway across the country to attend the convention. Their bus fare was paid by three progressive labor unions. One of the unions, the American Federation of Government Employees, was using a part of the $30,000 it has decided not to give to the Democratic Party this year.

"We're looking to put a little pressure on the Democratic Party," said Kenneth T. Blaylock, AFGE president. "They didn't hold a midterm convention because they didn't want the dissidents in the party -- labor, blacks, farmers -- raising their voices. Well, we're the bedrock of this party. Let them try to win without us."

This biracial frustration at the establishment, any establishment, coursed through the convention for three days, giving it electricity.

At one table, four farmers traded stories of trying to get a hearing from legislators on Capitol Hill, whom they had visited the day before. "[Sen.] Jesse Helms [(R-N.C.), Agriculture Committee chairman] nearly broke a neck trying to get away when he saw us coming," said Gene Trenthan, a Missouri soybean farmer. "They just don't give a damn," said Jake Batson of Galt, Mo.

"At least Jesse [Jackson] understands," said Leroy Neal, a bankrupt farmer from Wheeling, Mo. "Blacks have been oppressed so long, they know what it's like."

Minutes before, the farmers had been brought to their feet by a homily Jackson gave about how poor whites and poor blacks had been kept apart too long.

"We're like twins who are separated at birth," he told the group. "And we didn't know each other existed. And then we bumped into each other in the train station. Who is this stranger resembling me? Am I seeing double? We got the same needs. We got the same concerns. We got the same headaches. Same longings. Same country. Same joys. What happened to make us look at each other so funny? Someone above pulled us apart. But thanks be to God we have come together. And we are going to sit down now and have a family reunion."

It will, of course, take more than conventions and homilies to build an alliance. This one lacks the clear focus of the civil rights movement. It also confronts no end of potential internal conflicts. The Rainbow's agenda includes raising gas prices so energy workers won't suffer; fighting give-back wage settlements so union members won't suffer; raising price supports so farmers won't suffer; restoring social programs so blacks won't suffer.

Who pays? One obvious answer is all of the above, but Jackson isn't addressing that issue. He has always offered himself as the sort of protest figure who worries less about crafting programs, more about building constituencies. He recalled to reporters some advice he said Alabama Gov. George C. Wallace (D) once gave him: "You got to lay down the grass where the goats are." With neither political party focusing on the concerns of the poor, the soil seems fertile for Jackson's grass.

What remains for Jackson is the hard work of organizing. The Rainbow is scheduled to hold another convention a year from now, which will likely serve as the liftoff for Jackson's 1988 presidential campaign. And from Jackson's perspective, one of the best things about his thrust into new constituencies is that it may make him all the stronger in his base.