Are you workin' on a buildin'? Are you workin' on a buildin'?

John Jackson, rural bluesman and gravedigger from Rappahannock County, Va., wailed out the words across the South Lawn of the White House early yesterday evening in honor of Labor Day.

And the "laborers" who heard it as they sat on blankets, eating chicken and drinking beer, were treated not just to labor music and food but to a taste of President Carter working on his presidential reelection campaign.

"Well, its's better than last year's picnic," quipped Rachelle Horowitz, political director of the American Federation of Teachers, which originally was for Kennedy. "The beer's colder."

But the talk of labor and workers and Poland was all warmed up on the opening day of the fall campaign season.

"We look forward to returning to celebrate Labor Day with you over the next four years . . ." Lane Kirkland, president of the AFL-CIO, said on the podium on the South Lawn as the labor officials and their families broke into enthusiastic whoops and hollers. President and Mrs. Carter, who hosted this Labor Day picnic, broke into grins. Then Kirkland grinned, and continued, "if I am reelected."

Carter spoke at some length about the stunning workers' victory in Poland Saturday when that country granted workers the right to form independent trade unions. "Celebrating our own Labor Day holiday," said Carter, "Americans can look with gratitude and admiration at what the working people of Poland have accomplished. We are inspired by . . . their discipline, tenacity and personal courage. They have set an example for all who cherish freedom and human dignity. They have shown the world how to win a victory for labor, and how the hunger for human rights covers all the world.

"I am particularly glad that they achieved this without outside interference," he added.

Carter also presented special editions of a postage stamp commemorating the late George Meany, the long-time former president of the AFL-CIO, to Kirkland and to Meany's daughter, Eileen Meany Lee, who accepted for two other daughters as well.

Then Kirkland almost turned the picnic into a rousing worker's rally, when he declared, "If George Meany were here, I believe he would declare the fighting eagle on this stamp . . . is a Polish eagle. Long live the workers of Poland! Long live the cause of trade unionism the world over!"

It was mainly the Carter faithful of labor unions who were among the guests on the South Lawn -- 1,000 of them by White House estimate, a few hundred less than that count by visual estimate. They applauded hard when Carter said, "I'm determined no matter what obstacles to work with you toward labor law reform."

But few, if any people, from unions lukewarm or opposed to Carter were there. Few International Association of Machinists union people were there, and none was listed among the expected participants. Some who came were the less-than-enthusiastic labor converts.

"Obviously Carter has had some problems with our union," said Horowitz, standing on the dusky, muggy South Lawn. "But when he came to our convention in Chicago, he had demonstrated he knows more about problems of urban teachers. Certainly he's more pro-union than the other candidates. Carter needs the labor unions to get elected." She shrugged. "Our union is slowly getting more enthusiastic about Carter. If you asked me today how I felt about Carter versus Kennedy, I'd say Kennedy -- "

She was interrupted mid-sentence by Undersecretary of Labor Jack Gentry, who came running over. "Don't believe a word she says!" he shouted.

Then Ken Moffett, deputy director of the Federal Mediation Service appeared, beer in hand. "The way she switched, I couldn't believe it!" he said. "One day she was for Kennedy, the next day she was for Carter . . ."

Horowitz howled with laughter.

If everyone in labor is so pro-Carter, Moffett and Gentry were asked, then where was William Winpisinger, president of the International Association of Machinists, one of the first unions to back Kennedy publicly?

"He may be in the pool down there," Gentry said, looking across the lawn toward the Reflecting Pool.

Just then, Thomas R. Donahue, secretary-treasurer of the AFL-CIO, and an ardent supporter of Carter, approached Horowitz, who happens to be his wife of one year, and took her arm. "Don't believe a word these two say," Donahue warned a nearby listener.

Ray Marshall, secretary of labor, was there in shirtsleeves and enthusiastically cheered, especially when Carter presented him a lithograph by American artist Jacob Lawrence -- also there -- inscribed by the president to Marshall. Guests at the picnic also received the lithograph, created in a limited edition depicting Americans at work.

"I think the campaign's coming along," said Esther Peterson, Carter's consumer advisor. "Some [here] were disappointed. One said to me, 'Esther, how could you get behind him?' I said, I believe in him.' I've worked for enough presidents," she said ruefully. "I can tell the difference. I remember when Truman was running, my job at a big labor convention was to make sure his name wasn't mentioned for fear he'd be booed. And look -- he won."